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Kevin Federline is finally ready to do something about his recent weight gain. He's joining Celebrity Fit Club.

Britney Spears' ex-husband, 31, has officially signed on to participate in the next season of the VH1 reality show, the network confirms to

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The onetime-rapper -- who began filming his own reality show in August -- will be joined on Celebrity Fit Club by his other ex, Shar Jackson.

Jackson -- who is mother to two of Federline's children, Kori, 7, and Kaleb, 5 -- told Us that although the father of four is joining the show to lose weight, he doesn't mind his current size.

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"He doesn't care," she told Us at HBO's post-Emmys party in L.A. on Sunday about his burgeoning belly. "The beautiful thing about him is that he could care less."

Filming for the new season is underway and expected to air in early 2010.