Ninth Doctor (The Curse of Fatal Death)

An incarnation of the Doctor was glimpsed when the Eighth Doctor looked into the Tomorrow Windows. The Eighth Doctor described him and his female companion as "a listless-looking man [who] sat on a sofa beside a girl in a red dress in an unconvincing medieval dungeon". (PROSE: The Tomorrow Windows)

behind the scenes
Some interpret this description as the Doctor and Emma from NOTVALID: The Curse of Fatal Death.