Thirteenth Doctor (The Curse of Fatal Death)

The Thirteenth Doctor was the last, female incarnation of the Eighth Doctor in a possible future.

Biography
Her previous self regenerated after being hit by zectronic energy.

With the Doctor now regenerated into a woman, she was abandoned by her companion and lover, Emma, and "his" previous plans to marry her were cancelled. However, both the Doctor and the Master found each other much more attractive than before, and walked off together as a pair. (DW: The Curse of Fatal Death)

Behind the scenes

 * The inclusion of a woman as the thirteenth Doctor addressed long standing fan speculation that the Doctor could at some point change sexes while regenerating. The theory has been supported by many lines in the show and by Tom Baker.
 * As mentioned off-handily by the Eleventh Doctor himself (DW: The Doctor's Wife) it is indeed possible for Time Lords to change genders during a regeneration.
 * Fans had for years thought Joanna Lumley a candidate for the role of the Doctor, should they ever cast a woman in the role. She had earlier played Sapphire, a non-human time detective in the cult fantasy series Sapphire & Steel. The script gave her name as "The (Female) Thirteenth Doctor".
 * Like The Curse of Fatal Death, this incarnation of the Doctor was not considered canon.