Two more names have been added to the cast of the upcoming Doctor Who specials; model Gemma Chan and former Neighbours actor Peter O'Brien.

Chan was spotted filming on the streets of Newport last week with Lindsay Duncan and David Tennant. According to a casting website, castingcallpro.com, the model will play a character called Mia Bennett in the Christmas special.

According to the Sydney Daily Telegraph, O'Brien has confirmed that he will be filming Doctor Who at the end of this month. The former Casualty actor added that he will be playing a villain in the episode, part of David Tennant's final story, and it will be set on Mars. The Australian has previously worked with outgoing Who showrunner Russell T Davies on Queer As Folk.

So far the BBC has not confirmed the broadcast dates for any of the specials though it is believed that the first, Planet of the Dead, will air at Easter whilst the second will be shown on Christmas day. The very last episode will be screened in 2010, possibly New Year's Day.

• Doctor Who: Special No. 2 At Christmas