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Originally Posted by Teresa Edgerton  It may not mean that she'll be around for two whole seasons, though. Companions have left part way through a season, especially if the Doctor has more than one travelling with him at the same time. |
It would be advisable that one of the companions stay on for the regeneration whenever it happens. If a companion left and then the doctor regenerated, there would be no-one to react to the doctor's new appearance. It would almost be pointless!