text: Tennant was the 10th Doctor and Piper played his on-screen companion Rose Tyler in the BBC One show.
  Filming on the show, which will also star John Hurt, will start next week.
  Tennant's successor, Matt Smith - also in the special with his new assistant played by Jenna-Louise Coleman - says fans "will not be disappointed" by the 3D show, due to air on 23 November.
  Tennant and Piper have long been rumoured to be making a return for the special, which is being written by the show's executive producer and lead writer Steven Moffat.
  In January, Piper, appearing on the Graham Norton show, denied she would be appearing.
  "I wasn't asked, no," she said.
  "I think Matt Smith may have said, in passing or in jest, it would be nice.
  "I think maybe he said that and then it became something quite different, but no."
  Smith has said the show "manages to pay homage to everything - and look forward".
  "I read it and I clapped at the end. I think it's hilarious, it's epic and it's vast," he said.
  Moffat, meanwhile, has said he took special care to protect the secrets of the story.
  "One length I've gone to which is a really good security measure - I make sure I don't get a script, because I will lose it," he said.
  "I forbid people to hand me one. It's on my computer under lock and key."
  The first story of Doctor Who's 2013 run, The Bells of Saint John - described by Moffat as a "proper London thriller" - was screened on Saturday.
  Viewers saw the Doctor and new companion, Clara, played by Coleman, battling an evil entity in the world's wi-fi networks.
  Future episodes of the show, which is filmed in Cardiff, see the return of the Cybermen and old enemy the Ice Warriors, who last appeared during the Jon Pertwee era in 1974.
  Tennant starred in Doctor Who from 2005 to 2010 while Piper first appeared in 2005 opposite Christopher Eccleston, who played the ninth Doctor.
  She left the show in 2006 but returned for a number of episodes in 2008 as well as for Tennant's final episode, broadcast on 1 January 2010.
  The first episode of Doctor Who, An Unearthly Child, starring William Hartnell as the Timelord, was broadcast on 23 November 1963.
  As part of the anniversary events, the BBC will also broadcast An Adventure in Space and Time - a one-off drama looking at how the sci-fi show came to be made.
  Doctor Who: The Bells of Saint John will be shown on BBC One at 18:15 GMT on Saturday.
target: David Tennant and Billie Piper will appear in the 50th anniversary special of Doctor Who, the BBC has confirmed.