Their on-screen pairing as Rose and The Doctor made both Billie Piper and David Tennant household names.
So it was only fitting that new mother Billie should return to Doctor Who for Tennant 's final two episodes as the Timelord.
The 26-year-old actress joins an all-star cast for the emotional send-off of the popular star.
Billie's back: Actress Billie Piper, who once played The Doctor's companion Rose, reunites with David Tennant for his final appearance as the Timelord
Yesterday Tennant, dressed in the Doctor's signature suited style, was seen filming some of his last scenes from the sci-fi family drama.
He clutched the wall in apparent distress as he walked through air swirling with snow in a Cardiff Street.
Billie had earlier filmed scenes nearby, wrapped in scraf and gloves, and the two were seen chatting together in a break from filming.
Tense scenes: David Tennant walks through air swirling with snow as he films his last moments for a two-episode finale to his time in Doctor Who
The final two episodes will be broadcast over the Christmas period and will see Tennant regenerate as Matt Smith, who replaces him as the 11th Doctor.
It will also be the last of Russell T Davies’ four 2009 specials and signals the end of his five years as head writer.
He has breathed new life into the show and is bound to be planning a dramatic ending to his successful run on the show.
Will they be reunited? Rose and the Doctor had a love-affair on the show
Scottish writer Steven Moffat will take over as executive producer for a new series in 2010.
This week it was revealed that veteran actress Claire Bloom will appear in a cameo role as The Doctor's mother.
She joins comedienne Catherine Tate, who plays the Doctor's occasional companion Donna Noble and actor Bernard Cribbins, who plays Noble's grandfather Wilfred.
Wrapped up: Billie and Camille Coduri, who plays her onscreen mother Jackie Tyler, film scenes in Cardiff yesterday
Veteran actress June Whitfield - best known in recent times for playing Edina’s mother in Absolutely Fabulous - has also been spotted in the city.
She and Cribbins are long-time friends and appeared together on screen in Carry On Columbus in 1992.
Other new faces joining the filming are Hi-de-Hi! actor Barry Howard and Karl Collins, who plays Danny Glaze in The Bill.