Michelle Ryan and Lee Evans are to guest star in a Doctor Who Easter special.
Ryan, the former Eastenders star, will appear as Lady Christina de Souza who joins the Doctor on a trip which takes a "detour in danger", the BBC has said.
Comedian Evans will also appear in the special Planet of the Dead episode as a character called Malcolm whose life also becomes connected with the Doctor.
Ryan explained she was a "huge fan" of Doctor Who and "very excited" at the prospect at staring alongside David Tennant.
"It is such a fantastic show and I can't wait to get started," she said.
The show's executive producer and writer Russell T Davies added: "Michelle is one of the most sought after young actors in the country and we are delighted to announce that she will be joining the team.
"As always the script is being kept strictly under wraps however we can reveal that Lady Christina is a woman with a mysterious past who's going to have a huge impact on the Doctor."