Tennant played the Tenth Doctor between 2005-10, taking over from Christopher Eccleston who left the show after just one series.
Piper starred alongside both Tennant and Eccleston, leaving the series in 2007, although she guest starred in the fourth series.
Her character was last seen being left in a parallel universe with a half-human/half Timelord version of Tennant’s Doctor.
Piper and Tennant will appear in a special episode to mark the show’s anniversary which will also be celebrated with a special drama looking behind the scene’s of the show’s origins.
Filming for the story, which will also star John Hurt, starts next week.
The series returns tonight for a new 8-part run, starting with The Bells of St John. The story sees Matt Smith back in the Tardis and looking for Clara Oswald, played by Jenna-Louise Coleman.