Rachel Bilson and Adam Brody

Rachel Bilson has split from her 'The O.C.' co-star Adam Brody.The 25-year-old actress - who plays Summer in the hit US TV series - has been dating her on-screen boyfriend since 2004 but the couple have now decided to call time on their romance. A source told Life and Style Weekly magazine: "They've been done for a few weeks. They are just done. That's all there is to it. There's no drama. They just ended it."Rachel recently made her film debut starring along side Zach Braff in 'TheLast Kiss', while Adam has had roles in 'Mr and Mrs Smith' and 'Thank You For Smoking'. It seems Adam, 26, has already begun to enjoy his newly single status. Last week, he attended a launch party in Los Angeles alone and was spotted chatting to several women. A source said: "He was definitely acting like a single guy."Other Los Angeles clubbers say Adam has recently been spending a lot of time partying without Rachel and visiting clubs with groups of male friends.Representatives for both 'The O.C.' stars have so far refused to comment on the split.