An Estrati took the form of Peri Brown from the Sixth Doctor's recent memories while he aided her in recovering her spaceship.
After recovering her spaceship, she returned to her own plane of reality through the Gateway, and once safe within the Vortex she summoned her own people for help. (AUDIO: Harry Houdini's War)
The Sixth Doctor's companions from stage plays are difficult to assert with certainty, as different actors played the Doctor in the same play. If a medium is not mentioned, then this incarnation did not have companions who were original to that medium; it does not mean that this Doctor failed to appear in that medium.
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