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{{short description|Triple star system in the constellation Eridanus}}
{{other stars by Bayer designation|Omicron Eridani{{!}}ο Eridani}}
{{Starbox begin
| name = 40 Eridani / Keid
}}
{{Starbox image
 | image=
   {{Location mark
     | image=Eridanus constellation map.svg
     | float=center | width=280 | position=right
     | mark=Red circle.svg | mark_width=9 | mark_link=40 Eri
     | x%=37.0 | y%=16.5
   }}
 | caption=Location of 40 Eridani (circled)
}}
{{Starbox observe 3s
| epoch = [[J2000.0]]
| constell = [[Eridanus (constellation)|Eridanus]]
| component1 = 40 Eridani A
| ra1 = {{RA|04|15|16.31962}}<ref name="GaiaDR3a"/>
| dec1 = {{DEC|−07|39|10.3308}}<ref name="GaiaDR3a"/>
| appmag_v1 = 4.43<ref name="hip" />
| component2 = 40 Eridani B
| ra2 = {{RA|04|15|21.79572}}<ref name="GaiaDR3b"/>
| dec2 = {{DEC|−07|39|29.2040}}<ref name="GaiaDR3b"/>
| appmag_v2 = 9.52<ref name="gj">[[Gliese Catalogue of Nearby Stars]], preliminary 3rd ed., 1991. [[Centre de Données astronomiques de Strasbourg|CDS]] ID [http://vizier.u-strasbg.fr/viz-bin/Cat?V/70A V/70A].</ref>
| component3 = 40 Eridani C
| ra3 = {{RA|04|15|21.53600}}<ref name="GaiaDR3c"/>
| dec3 = {{DEC|−07|39|20.6946}}<ref name="GaiaDR3c"/>
| appmag_v3 = 11.17<ref name="gj" />
}}
{{Starbox character
| component1 = 40 Eridani A
| class = K0.5V<ref name=nstars>{{cite journal|bibcode=2006AJ....132..161G|title=Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 pc-The Southern Sample|journal=The Astronomical Journal|volume=132|issue=1|pages=161–170|last1=Gray|first1=R. O.|last2=Corbally|first2=C. J.|last3=Garrison|first3=R. F.|last4=McFadden|first4=M. T.|last5=Bubar|first5=E. J.|last6=McGahee|first6=C. E.|last7=O'Donoghue|first7=A. A.|last8=Knox|first8=E. R.|year=2006|doi=10.1086/504637|arxiv = astro-ph/0603770 |s2cid=119476992}}</ref>
| r-i = <!--R-I color-->
| v-r = <!--V-R color-->
| b-v = +0.82<ref name="hip" />
| u-b = +0.45<ref name="gj" />
| variable =
}}
{{Starbox character|no_heading=y
| component1 = 40 Eridani B
| class = DA4<ref name="gj" />
| r-i = <!--R-I color-->
| v-r = <!--V-R color-->
| b-v = +0.03<ref name="gj" />
| u-b = +0.45<ref name="gj" />
| variable = <!--Variability type-->
| component2 = 40 Eridani C
| class2 = M4.5eV<ref name="gc">[[General Catalogue of Trigonometric Parallaxes]], 4th ed., 1995. [[Centre de Données astronomiques de Strasbourg|CDS]] ID [http://vizier.u-strasbg.fr/viz-bin/Cat?I/238A I/238A].</ref>
| r-i2 = <!--R-I color-->
| v-r2 = <!--V-R color-->
| b-v2 = +1.67<ref name="gj" />
| u-b2 = +0.83<ref name="gj" />
| variable2 = [[Flare star]]<ref name=gcvs>{{cite journal|bibcode=2009yCat....102025S|title=VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)|journal=VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S|volume=1|pages=B/gcvs|display-authors=etal|last1=Samus|first1=N. N.|last2=Durlevich|first2=O. V.|year=2009}}</ref>
}}
{{Starbox astrometry
| component1 = 40 Eridani A
| radial_v = {{val|−42.47|0.12}}<ref name="GaiaDR3a"/>
| prop_mo_ra = −2,240.085
| prop_mo_dec = −3,421.809
| pm_footnote = <ref name="GaiaDR3a"/>
| parallax = 199.6080
| p_error = 0.1208
| parallax_footnote = <ref name="GaiaDR3a"/>
| space_v_u = <!--Space velocity (U), σu cos v w (in km/s)-->
| space_v_v = <!--Space velocity (V), σv cos w (in km/s)-->
| space_v_w = <!--Space velocity (W), σw (in km/s)-->
| absmag_v = 5.93<ref name=Holmberg2009/>
}}
{{Starbox astrometry|no_heading=y
| component1 = 40 Eridani B
| radial_v = −21<ref name=rv>{{cite journal|bibcode=1967IAUS...30...57E|title=The Revision of the General Catalogue of Radial Velocities|journal=Determination of Radial Velocities and Their Applications|volume=30|pages=57|last1=Evans|first1=D. S|year=1967}}</ref>
| prop_mo_ra = −2,236.169
| prop_mo_dec = −3,338.955
| pm_footnote = <ref name="GaiaDR3b"/>
| parallax = 199.6911
| p_error = 0.0512
| parallax_footnote = <ref name="GaiaDR3b"/>
| space_v_u = <!--Space velocity (U), σu cos v w (in km/s)-->
| space_v_v = <!--Space velocity (V), σv cos w (in km/s)-->
| space_v_w = <!--Space velocity (W), σw (in km/s)-->
| absmag_v =
| component2 = 40 Eridani C
| radial_v2 = {{val|−44.06|0.20}}<ref name="GaiaDR3c"/>
| prop_mo_ra2 = −2,247.183
| prop_mo_dec2 = −3,409.824
| pm_footnote2 = <ref name="GaiaDR3c"/>
| parallax2 = 199.4516
| p_error2 = 0.0692
| parallax_footnote2 = <ref name="GaiaDR3c"/>
| space_v_u2 = <!--Space velocity (U), σu cos v w (in km/s)-->
| space_v_v2 = <!--Space velocity (V), σv cos w (in km/s)-->
| space_v_w2 = <!--Space velocity (W), σw (in km/s)-->
| absmag_v2 =
}}
{{Starbox visbin
| reference = <!--Reference-->
| primary = 40 Eridani A
| name = 40 Eridani BC
| period = ~8,000<ref name="heintz" />
| axis =
| axis_unitless = ~400<ref name="ss" /> [[Astronomical unit|AU]]
| eccentricity = <!--Eccentricity-->
| inclination = <!--Inclination (in degrees)-->
| node = <!--Longitude of node (in degrees)-->
| periastron = <!--Periastron epoch-->
| periarg = <!--Argument of periastron (in degrees)-->
}}
{{Starbox orbit
| reference = <ref name=Mason2017/>
| primary = 40 Eridani B
| name = 40 Eridani C
| period = {{Val|230.30|0.68}}
| axis = {{Val|6.930|0.050}}
| axis_unitless = ~35 [[Astronomical unit|AU]]
| eccentricity = {{Val|0.4294|0.0027}}
| inclination = {{Val|107.56|0.29}}
| node = {{Val|151.44|0.12}}
| periastron = {{Val|1847.7|1.1}}
| periarg = {{Val|318.4|1.1}}
}}
{{Starbox detail
| source = <!--[source url]-->
| component1 = 40 Eridani A
| mass = {{val|0.78|0.08}}<ref name="exoplanet"/>
| radius = {{val|0.804|0.006}}<ref name=Rains2020>{{cite journal
 | title=Precision angular diameters for 16 southern stars with VLTI/PIONIER
 | display-authors=1 | last1=Rains | first1=Adam D.
 | last2=Ireland | first2=Michael J. | last3=White | first3=Timothy R.
 | last4=Casagrande | first4=Luca | last5=Karovicova | first5=I.
 | journal=Monthly Notices of the Royal Astronomical Society
 | volume=493 | issue=2 | pages=2377–2394 | date=April 2020
 | arxiv=2004.02343 | bibcode=2020MNRAS.493.2377R 
 | doi=10.1093/mnras/staa282 | doi-access=free }}</ref>
| gravity = {{val|4.35|0.1}}<ref name=Rains2020/>
| luminosity = {{Val|0.4|0.01}}<ref name=Rains2020/>
| temperature = {{val|5126|30}}<ref name=Rains2020/>
| metal_fe = {{val|−0.36|0.02}}<ref name=Rains2020/>
| rotational_velocity = {{nowrap|1.23 ± 0.28}}<ref name="Díaz2018"/>
| rotation = ~37–43<ref name="Díaz2018"/>&nbsp;days
| age_gyr = {{val|6.9|4.7}}<ref name="exoplanet"/>
}}
{{Starbox detail|no_heading = y
| component1 = 40 Eridani B
| mass = {{Val|0.573|0.018}}<ref name=Mason2017/>
| radius = 0.014<ref name="hippmr">{{cite journal|bibcode=1998ApJ...494..759P|title=Testing the White Dwarf Mass-Radius Relation with HIPPARCOS|journal=The Astrophysical Journal|volume=494|issue=2|pages=759|last1=Provencal|first1=J. L.|last2=Shipman|first2=H. L.|last3=Høg|first3=Erik|last4=Thejll|first4=P.|year=1998|doi=10.1086/305238|citeseerx=10.1.1.44.7051|s2cid=122724497 }}</ref>
| gravity = <!--Surface gravity (given as the base 10 logarithm expressed in cgs units)-->
| luminosity = 0.013<ref name="keid"/>
| temperature = 16,500<ref name="das">{{cite journal|bibcode=1997ApJ...488..375F|title=The Temperature Scale and Mass Distribution of Hot DA White Dwarfs|journal=The Astrophysical Journal|volume=488|issue=1|pages=375–396|last1=Finley|first1=David S.|last2=Koester|first2=Detlev|last3=Basri|first3=Gibor|year=1997|doi=10.1086/304668|s2cid=120006763 |doi-access=free}}</ref>
| component2 = 40 Eridani C
| mass2 = {{val|0.271|0.001}}<ref name=carmenes21/>
| radius2 = {{val|0.298|0.009}}<ref name=carmenes21/>
| gravity2 = ~5.5<ref name=carmenes21/>
| luminosity2 = {{val|0.00651|0.00013|fmt=none}}<ref name=carmenes21/>
| temperature2 = 3,100<ref name="ir">{{cite journal|bibcode=1983ApJS...53..643J|title=Predicted infrared brightness of stars within 25 parsecs of the sun|journal=Astrophysical Journal Supplement Series|volume=53|pages=643|last1=Johnson|first1=H. M.|last2=Wright|first2=C. D.|year=1983|doi=10.1086/190905|doi-access=free}}</ref>
| age_gyr2=5.6<ref name="MH08">{{cite journal
 | author=Mamajek, Eric E.
 | author2=Hillenbrand, Lynne A.
 | title=Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics
 | journal=The Astrophysical Journal | volume=687 | issue=2
 | pages=1264–1293 |date=November 2008
 | doi=10.1086/591785 | bibcode=2008ApJ...687.1264M |arxiv = 0807.1686 | s2cid=27151456
 }}</ref>&nbsp;Gyr<br>1.8<ref name=Bond_et_al_2017/>
}}
{{Starbox catalog
 | names = {{odlist | B=ο<sup>2</sup>&nbsp;Eri | F=40&nbsp;Eri | ADS=3093 | CCDM=J04153-0739 | GCTP=945 | GJ=166 }}
 | component1 = A
 | names1 = {{odlist | name=Keid<ref name="IAU-LSN"/> | HD=26965 | HIP=19849 | HR=1325 | LHS=23 | LTT=1907 | SAO=131063 | BD=−07° 780 }}
 | component2 = B
 | names2 = {{odlist | BD=−07° 781 | G=160-060 | HD=26976 | LHS=24 | LTT=1908 | SAO=131065 }}
 | component3 = C
 | names3 = {{odlist | V=DY&nbsp;Eri | BD=-07°781&nbsp;C | LHS=25 | LTT=1909 }}
}}
{{Starbox reference
 | Simbad = 40+Eri|sn=A
 | Simbad3 = *+omi02+Eri+B|sn3=B
 | Simbad4 = *+omi02+Eri+C|sn4=C
 | ARICNS = <!--Code-->
}}
{{Starbox end}}

'''40 Eridani''' is a [[triple star system]] in the [[constellation]] of [[Eridanus (constellation)|Eridanus]], abbreviated '''40 Eri'''. It has the [[Bayer designation]] '''Omicron<sup>2</sup> Eridani''', which is [[Romanization of Greek|Latinized]] from '''ο<sup>2</sup> Eridani''' and abbreviated Omicron<sup>2</sup> Eri or '''ο<sup>2</sup> Eri'''. Based on [[stellar parallax|parallax]] measurements taken by the [[Gaia (spacecraft)|Gaia]] mission, it is about 16.3 [[light-year]]s from the [[Sun]].

The primary star of the system, designated 40 Eridani A and named '''Keid''',<ref name="IAU-LSN">{{cite web | url=https://www.iau.org/public/themes/naming_stars/ | title=Naming Stars |publisher=IAU.org |access-date=16 December 2017}}</ref> is easily visible to the naked eye. It is orbited by a [[binary star|binary pair]] whose two components are designated 40 Eridani B and C, and which were discovered on January 31, 1783, by [[William Herschel]].<ref name=":0">{{cite journal | jstor=106749| title=Catalogue of Double Stars. By William Herschel, Esq. F. R. S| journal=Philosophical Transactions of the Royal Society of London| volume=75| pages=40–126| last1=Herschel| first1=William| year=1785| bibcode=1785RSPT...75...40H| doi=10.1098/rstl.1785.0006| s2cid=186209747}}</ref>{{rp|p73}} It was again observed by [[Friedrich Georg Wilhelm von Struve|Friedrich Struve]] in 1825 and by [[Otto Wilhelm von Struve|Otto Struve]] in 1851.<ref name="heintz">{{cite journal|bibcode=1974AJ.....79..819H|title=Astrometric study of four visual binaries|journal=Astronomical Journal|volume=79|pages=819|last1=Heintz|first1=W. D.|year=1974|doi=10.1086/111614}}</ref><ref>{{cite journal|bibcode=1926BAN.....3..128V|title=The orbit and the masses of 40 Eridani BC|journal=Bulletin of the Astronomical Institutes of the Netherlands|volume=3|pages=128|last1=Van Den Bos|first1=W. H.|year=1926}}</ref>

In 1910, it was discovered that although component B was a faint star, it was white in color. This meant that it had to be a small star; in fact it was a [[white dwarf]], the first discovered.<ref name="schatzman">''White Dwarfs'', E. Schatzman, Amsterdam: North-Holland, 1958. <sup>, p.&nbsp;1</sup></ref> Although it is neither the closest white dwarf, nor the brightest in the night sky, it is by far the easiest to observe; it is nearly three magnitudes brighter than [[Van Maanen 2|Van Maanen's Star]], the nearest solitary white dwarf, and unlike the companions of [[Procyon]] and [[Sirius]] it is not swamped in the glare of a much brighter primary.<ref name="keid">[http://www.astro.uiuc.edu/~kaler/sow/keid.html Keid] {{webarchive|url=https://web.archive.org/web/20070514055905/http://www.astro.uiuc.edu/~kaler/sow/keid.html |date=2007-05-14 }}, Jim Kaler, STARS web page, accessed 15/5/2007, 10/12/2011.</ref>

==Nomenclature==
40 Eridani is the system's [[Flamsteed designation]] and ο²&nbsp;Eridani ([[Latinisation of names|Latinised]] to Omicron<sup>2</sup>&nbsp;Eridani) its [[Bayer designation]]. The designations of the sub-components – {{nobr|40 Eridani A,}} B and C – derive from the convention used by the ''Washington Multiplicity Catalog'' (WMC) for [[multiple star system]]s, and adopted by the [[International Astronomical Union]] (IAU).<ref name="planetnaming">{{cite arXiv  |last1=Hessman |first1=F.V. |last2= Dhillon |first2=V.S. |last3=Winget |first3=D.E. |last4=Schreiber |first4=M.R. |last5=Horne |first5=K. |last6=Marsh |first6=T.R. |last7=Guenther |first7=E. |last8=Schwope |first8=A. |last9=Heber |first9=U. |display-authors=6 |year=2010 |title=On the naming convention used for multiple star systems and extrasolar planets |eprint=1012.0707 |class=astro-ph.SR}}</ref> {{nobr|40 Eridani C}} also bears the [[variable star designation]] DY&nbsp;Eridani.

The system bore the traditional name ''Keid'' derived from the [[Arabic]] word قيض ({{transl|ar|alqayḍ}}) meaning "the [[eggshell]]s," alluding to its neighbour [[Omicron1 Eridani|Beid]] (Arabic "egg").<ref>{{Cite web|url=http://stars.astro.illinois.edu/sow/beid.html|title=Beid|website=stars.astro.illinois.edu}}</ref> In 2016, the IAU organized a [[IAU Working Group on Star Names|Working Group on Star Names]] (WGSN)<ref name="WGSN">
{{cite web
 | title=Working Group on Star Names (WGSN)
 | series=Working Groups
 | publisher=[[International Astronomical Union]]
 | url=https://www.iau.org/science/scientific_bodies/working_groups/280/
 | access-date=22 May 2016
}}
</ref>
to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire [[multiple star|multiple systems]].<ref name="TriRpt18">
{{cite report
 |title=Working Group on Star Names Triennial Report (2015-2018)
 |year=2018
 |publisher=[[International Astronomical Union]]
 |page=5
 |url=https://www.iau.org/static/science/scientific_bodies/working_groups/280/wg-starnames-triennial-report-2015-2018.pdf
 |access-date=2018-07-14
}}
</ref>
It approved the name ''Keid'' for the component {{nobr|40 Eridani A}} on 12&nbsp;September 2016 and it is now so included in the List of IAU-approved Star Names.<ref name="IAU-LSN"/>

==Properties==
[[File:40 Eridani.png|thumb|left|Amateur photo of 40 Eridani]]
40 Eridani A is a [[main sequence|main-sequence]] [[dwarf star|dwarf]] of [[spectral type]] K1, 40 Eridani B is a 9th magnitude [[white dwarf]] of spectral type DA4, and 40 Eridani C is an 11th magnitude [[red dwarf]] [[flare star]] of spectral type M4.5e. When component B was a main-sequence star, it is thought to have been the most massive member of the system, but ejected most of its mass before it became a white dwarf.<ref name=Bond_et_al_2017/> B and C [[orbit]] each other approximately 400 [[astronomical unit|AU]] from the primary star, A.<ref name="ss" /> Their orbit has a semimajor axis of 35 [[astronomical unit|AU]] and is rather elliptical with an [[orbital eccentricity]] of 0.410).<ref name="heintz" />

As seen from the 40 Eridani system, the Sun is a 3.4-magnitude star in [[Hercules (constellation)|Hercules]], near the border with [[Serpens Caput]].<ref group="note">From 40 Eridani the Sun would appear on the diametrically opposite side of the sky at the coordinates RA={{RA|16|15|16.32}}, Dec={{DEC|07|39|10.34}}, which is located near the border of [[Hercules (constellation)]] and [[Serpens Caput]], the closest bright star being [[Alpha Serpentis]]. The absolute magnitude of the Sun is 4.85, so, at a distance of 5.04 parsecs, the Sun would have an apparent magnitude <math>\begin{smallmatrix}m\ =\ M_v\ +\ 5\cdot((\log_{10}\ 5.04)\ -\ 1)\ =\ 3.36\end{smallmatrix}</math>.</ref>

===Potential for life===
The [[habitable zone]] of {{nobr|40 Eridani A,}} where a planet could exist with liquid water, is near 0.68&nbsp;[[astronomical unit|{{sc|AU}}]] from A. At this distance a planet would complete a revolution in 223&nbsp;Earth days (according to the third of [[Kepler's laws of planetary motion|Kepler's laws]]) and {{nobr|40 Eridani A}} would appear nearly 20%<ref name="radiusperspective" group="note">From <math>\ h = \frac{\ a\ }{d} \,,</math> where {{mvar|h}} is the apparent height, {{mvar|d}} is the distance of the object, and {{mvar|a}} is the actual size of the object.</ref> wider than the Sun does on Earth. An observer on a planet in the {{nobr|40 Eridani A}} system would see the B-C pair as unusually bright white and reddish-orange stars in the night sky – [[apparent magnitude|magnitudes]] −8 and −6, slightly brighter than the appearance of [[Venus]] seen from Earth as the [[Hesperus|evening star]].

It is unlikely that habitable planets exist around {{nobr|40 Eridani B}} because they would have been sterilized by its evolution into a white dwarf. As for {{nobr|40 Eridani C}}, it is prone to flares, which cause large momentary increases in the emission of [[X-ray]]s as well as visible light. This would be lethal to [[Earth]]-type life on planets near the flare star.<ref name="ss">{{cite web |title={{nobr|40 Eridani 3}} (Omicron² Eridani) |website=solstation.com |url=http://www.solstation.com/stars/40erida3.htm |access-date=2018-02-06 |archive-url=https://web.archive.org/web/20230930091651/http://www.solstation.com/stars/40erida3.htm |archive-date=30 September 2023 |url-status=dead}}</ref>

==Search for planets==
40 Eridani A shows periodic [[Doppler spectroscopy|radial velocity variations]], which were suggested to be caused by a planetary companion. The 42-day period is close to the stellar rotation period, which made the possible planetary nature of the signal difficult to confirm.<ref name="Díaz2018"/> A 2018 study found that most evidence supports a planetary origin for the signal,<ref name="exoplanet"/> but this has remained controversial, with a 2021 study characterizing the signal as a false positive.<ref name=Rosenthal2021/> As of 2022, the cause of the radial velocity variations remained inconclusive.<ref name="Zhao2022"/>

Further studies in 2023<ref name="Laliotis2023"/>{{rp|pages=23-24,44}} and 2024 concluded that the radial velocity signal very likely does originate from stellar activity, and not from a planet.<ref name="Burrows2024"/>

The candidate planet would have had a [[minimum mass]] of {{val|8.47|0.47|ul=Earth mass}}, and lie considerably interior to the habitable zone, receiving nine times more stellar flux than Earth, which is an even greater amount than [[Mercury (planet)|Mercury]], the innermost planet in the Solar System, on average receives from the Sun.<ref name="exoplanet"/>

==In fiction==
In the ''[[Star Trek]]'' franchise, the planet [[Vulcan (Star Trek)|Vulcan]] orbits 40 Eridani A.<ref>{{cite web |url=http://www.projectrho.com/vulsun.htm |title=Vulcan's Sun |author1=Sallie Baliunas |author-link1=Sallie Baliunas |author2=Gene Roddenberry |author-link2=Gene Roddenberry |access-date=2011-04-21 |display-authors=etal |archive-date=August 29, 2009 |archive-url=https://web.archive.org/web/20090829142538/http://www.projectrho.com/vulsun.htm |url-status=live}}</ref> Vulcan has been referenced in relation to the real-life search for exoplanets in this system.<ref>{{Cite news|url=https://www.science.org/content/article/spock-s-home-world-has-been-discovered-sort|title=Spock's home world has been discovered (sort of)|date=2018-09-18|journal=[[Science (journal)|Science]]|access-date=2018-09-20|language=en |doi=10.1126/science.aav4653 |last1=Clery |first1=Daniel }}</ref><ref name="Burrows2024"/> The hypothetical planet 40 Eridani A b is also mentioned in the book [[Project Hail Mary]] as the home of the eponymous Eridian species.<ref>{{Cite book |last=Weir |first=Andy |title=Project Hail Mary |date=2021 |publisher=Ballantine Books |isbn=978-0-593-13520-4 |edition= |location=New York |pages=238}}</ref>

==Notes==
{{reflist|group=note|2}}

==References==
{{Reflist|2|refs=

<ref name="GaiaDR3a">{{Cite Gaia DR3|3195919528989223040}}</ref>
<ref name="GaiaDR3b">{{Cite Gaia DR3|3195919254111315712}}</ref>
<ref name="GaiaDR3c">{{Cite Gaia DR3|3195919254111314816}}</ref>

<ref name="hip">{{Cite journal| first1=F. | last1=van Leeuwen |year=2007 | title=Validation of the new Hipparcos reduction | journal=Astronomy and Astrophysics | volume=474 | issue=2 | pages=653–664 | bibcode=2007A&A...474..653V | doi=10.1051/0004-6361:20078357 |arxiv = 0708.1752| s2cid=18759600 }}</ref>

<ref name=Holmberg2009>{{citation
 | display-authors=1 | last1=Holmberg | first1=J.
 | last2=Nordström | first2=B. | last3=Andersen | first3=J.
 | title=The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics
 | volume=501 | issue=3 |date=July 2009 | pages=941–947
 | journal=Astronomy and Astrophysics
 | doi=10.1051/0004-6361/200811191 | bibcode=2009A&A...501..941H
 | arxiv=0811.3982 | s2cid=118577511 | postscript=. }}</ref>

<ref name=Mason2017>{{citation
 | title=Binary Star Orbits. V. The Nearby White Dwarf/Red Dwarf Pair 40 Eri BC
 | last1=Mason | first1=Brian D. | last2=Hartkopf | first2=William I.
 | last3=Miles | first3=Korie N. | display-authors=1
 | journal=The Astronomical Journal
 | volume=154 | issue=5 | id=200 | pages=9 | date=November 2017
 | doi=10.3847/1538-3881/aa803e | postscript=.
 | bibcode=2017AJ....154..200M | arxiv=1707.03635 | s2cid=119080080 | doi-access=free }}</ref>

<ref name="Díaz2018">{{Cite journal |arxiv=1801.03970 |last1=Díaz |first1=Matías R. |last2=Jenkins |first2=James S. |last3=Tuomi |first3=Mikko |last4=Butler |first4=R. Paul |last5=Soto |first5=Maritza G. |last6=Teske |first6=Johanna K. |last7=Feng |first7=Fabo |last8=Shectman |first8=Stephen A. |last9=Arriagada |first9=Pamela |last10=Crane |first10=Jeffrey D. |last11=Thompson |first11=Ian B. |last12=Vogt |first12=Steven S. |title=The test case of HD26965: Difficulties disentangling weak Doppler signals from stellar activity |url=https://www.researchgate.net/publication/322498205 |journal=The Astronomical Journal |volume=155 |issue=3 |pages=126 |year=2018 |bibcode=2018AJ....155..126D |doi=10.3847/1538-3881/aaa896 |s2cid=119404519 |doi-access=free }}</ref>

<ref name=carmenes21>{{Citation |last1=Cifuentes |first1=C. |last2=Caballero |first2=J. A. |last3=Cortés-Contreras |first3=M. |last4=Montes |first4=D. |last5=Abellán |first5=F. J. |last6=Dorda |first6=R. |last7=Holgado |first7=G. |last8=Zapatero Osorio |first8=M. R. |last9=Morales |first9=J. C. |last10=Amado |first10=P. J. |last11=Passegger |first11=V. M. |last12=Quirrenbach |first12=A. |last13=Reiners |first13=A. |last14=Ribas |first14=I. |last15=Sanz-Forcada |first15=J. |date=2020-10-01 |title=CARMENES input catalogue of M dwarfs. V. Luminosities, colours, and spectral energy distributions |bibcode=2020A&A...642A.115C |journal=Astronomy and Astrophysics |volume=642 |pages=A115 |doi=10.1051/0004-6361/202038295 |issn=0004-6361|arxiv=2007.15077 }}</ref>

<ref name="exoplanet">{{cite journal | doi=10.1093/mnras/sty1933 |bibcode=2018MNRAS.480.2411M | arxiv=1807.07098 |title=The first super-Earth Detection from the High Cadence and High Radial Velocity Precision Dharma Planet Survey |journal=Monthly Notices of the Royal Astronomical Society |volume=480 |issue=2 |pages=2411 |year=2018 |last1=Ma |first1=Bo |last2=Ge |first2=Jian |last3=Muterspaugh |first3=Matthew |last4=Singer |first4=Michael A |last5=Henry |first5=Gregory W |last6=González Hernández |first6=Jonay I |last7=Sithajan |first7=Sirinrat |last8=Jeram |first8=Sarik |last9=Williamson |first9=Michael |last10=Stassun |first10=Keivan |last11=Kimock |first11=Benjamin |last12=Varosi |first12=Frank |last13=Schofield |first13=Sidney |last14=Liu |first14=Jian |last15=Powell |first15=Scott |last16=Cassette |first16=Anthony |last17=Jakeman |first17=Hali |last18=Avner |first18=Louis |last19=Grieves |first19=Nolan |last20=Barnes |first20=Rory |last21=Gilda |first21=Sankalp |last22=Grantham |first22=Jim |last23=Stafford |first23=Greg |last24=Savage |first24=David |last25=Bland |first25=Steve |last26=Ealey |first26=Brent |doi-access=free | display-authors=1 }}</ref>

<ref name=Rosenthal2021>{{cite journal |arxiv=2105.11583|doi=10.3847/1538-4365/abe23c|title=The California Legacy Survey. I. A Catalog of 178 Planets from Precision Radial Velocity Monitoring of 719 Nearby Stars over Three Decades|year=2021|last1=Rosenthal|first1=Lee J.|last2=Fulton|first2=Benjamin J.|last3=Hirsch|first3=Lea A.|last4=Isaacson|first4=Howard T.|last5=Howard|first5=Andrew W.|last6=Dedrick|first6=Cayla M.|last7=Sherstyuk|first7=Ilya A.|last8=Blunt|first8=Sarah C.|last9=Petigura|first9=Erik A.|last10=Knutson|first10=Heather A.|last11=Behmard|first11=Aida|last12=Chontos|first12=Ashley|last13=Crepp|first13=Justin R.|last14=Crossfield|first14=Ian J. M.|last15=Dalba|first15=Paul A.|last16=Fischer|first16=Debra A.|last17=Henry|first17=Gregory W.|last18=Kane|first18=Stephen R.|last19=Kosiarek|first19=Molly|last20=Marcy|first20=Geoffrey W.|last21=Rubenzahl|first21=Ryan A.|last22=Weiss|first22=Lauren M.|last23=Wright|first23=Jason T.|journal=The Astrophysical Journal Supplement Series|volume=255|issue=1|page=8|bibcode=2021ApJS..255....8R|s2cid=235186973 |doi-access=free }}</ref>

<ref name="Zhao2022">{{cite journal |arxiv=2201.10639|doi=10.3847/1538-3881/ac5176 |title=The EXPRES Stellar Signals Project II. State of the Field in Disentangling Photospheric Velocities |year=2022 |last1=Zhao |first1=Lily L. |last2=Fischer |first2=Debra A. |last3=Ford |first3=Eric B. |last4=Wise |first4=Alex |last5=Cretignier |first5=Michaël |last6=Aigrain |first6=Suzanne |last7=Barragan |first7=Oscar |last8=Bedell |first8=Megan |last9=Buchhave |first9=Lars A. |last10=Camacho |first10=João D. |last11=Cegla |first11=Heather M. |last12=Cisewski-Kehe |first12=Jessi |last13=Collier Cameron |first13=Andrew |last14=De Beurs |first14=Zoe L. |last15=Dodson-Robinson |first15=Sally |last16=Dumusque |first16=Xavier |last17=Faria |first17=João P. |last18=Gilbertson |first18=Christian |last19=Haley |first19=Charlotte |last20=Harrell |first20=Justin |last21=Hogg |first21=David W. |last22=Holzer |first22=Parker |last23=John |first23=Ancy Anna |last24=Klein |first24=Baptiste |last25=Lafarga |first25=Marina |last26=Lienhard |first26=Florian |last27=Maguire-Rajpaul |first27=Vinesh |last28=Mortier |first28=Annelies |last29=Nicholson |first29=Belinda |last30=Palumbo |first30=Michael L. |journal=The Astronomical Journal |volume=163 |issue=4 |page=171 |bibcode=2022AJ....163..171Z |s2cid=246285610 |display-authors=1 |doi-access=free }}</ref>

<ref name=Bond_et_al_2017>{{cite journal
 | title=Astrophysical Implications of a New Dynamical Mass for the Nearby White Dwarf 40 Eridani B
 | last1=Bond | first1=Howard E. | last2=Bergeron | first2=P. | last3=Bédard | first3=A.
 | journal=The Astrophysical Journal
 | volume=848 | issue=1 | id=16
 | date=October 2017 | page=16 | doi=10.3847/1538-4357/aa8a63
 | arxiv=1709.00478 | bibcode=2017ApJ...848...16B | s2cid=119257982 | doi-access=free }}</ref>

<ref name="Laliotis2023">{{cite journal |last1=Laliotis |first1=Katherine |last2=Burt |first2=Jennifer A. |display-authors=etal |date=February 2023 |title=Doppler Constraints on Planetary Companions to Nearby Sun-like Stars: An Archival Radial Velocity Survey of Southern Targets for Proposed NASA Direct Imaging Missions |journal=[[The Astronomical Journal]] |volume= 165|issue= 4|page=176 |doi= 10.3847/1538-3881/acc067|arxiv=2302.10310 |bibcode=2023AJ....165..176L |doi-access=free }}</ref>

<ref name="Burrows2024">{{cite journal |last1=Burrows |first1=Abigail |last2=Halverson |first2=Samuel |display-authors=etal |date=April 2024 |title=The Death of Vulcan: NEID Reveals That the Planet Candidate Orbiting HD 26965 Is Stellar Activity |journal=[[The Astronomical Journal]] |volume=167 |issue=5 |page=243 |doi=10.3847/1538-3881/ad34d5 |doi-access=free |arxiv=2404.17494|bibcode=2024AJ....167..243B }}</ref>

}}

==External links==
* [https://web.archive.org/web/20070514055905/http://www.astro.uiuc.edu/~kaler/sow/keid.html Keid] at Jim Kaler's STARS.
* [http://www.stellar-database.com/Scripts/search_star.exe?ID=175600 Omicron(2) Eridani] entry at the Internet Stellar Database.

{{Sky|04|15|16.32|-|07|39|10.34|16.45}}
{{nearest systems|3}}
{{Stars of Eridanus}}
{{Portal bar|Astronomy|Stars|Spaceflight|Outer space|Solar System}}
{{DEFAULTSORT:40 Eridani}}
<!-- Properties -->
[[Category:Emission-line stars]]
[[Category:K-type main-sequence stars]]
[[Category:M-type main-sequence stars]]
[[Category:White dwarfs]]
[[Category:Solar-type stars|Eridani, 40]]
[[Category:Triple star systems]]
[[Category:Hypothetical planetary systems]]
<!-- Location/catalogues -->
[[Category:Stars with proper names|Keid]]
[[Category:Eridanus (constellation)]]
[[Category:Bayer objects|Eridani, Omicron2]]
[[Category:Bright Star Catalogue objects|1325]]
[[Category:Durchmusterung objects]]
[[Category:Flamsteed objects|Eridani, 40]]
[[Category:Gliese and GJ objects|0166]]
[[Category:Henry Draper Catalogue objects|026965]]
[[Category:Hipparcos objects|019849]]
[[Category:Objects with variable star designations|Eridani, DY]]

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