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The producers of Rihanna's "Birthday Cake" remix promised that her collaborator would be a shocker, and jaws dropped when it was revealed on Monday night that the mystery man was none other than Chris Brown.

Brown tweeted a link to the new music, along with a couple words from the blush-worthy track - "Cake, cake, cake!" - which Rihanna retweeted along with some additional lyrics, "#RihannaNavy Come and put ya name on it!!!!!"  Brown also shared a link revealing that Rihanna had returned the favor, reuniting - at least on wax - with her former beau on a re-do of his Fortune single "Turn Up the Music."

Da Internz, who produced "Birthday Cake" along with The-Dream, had earlier denied that Brown would appear on Ri-Ri's revamped song.  But the rumors that the collabo was happening only grew on Monday morning when Brown  tweeted, "HAPPY BIRTHDAY ROBYN!," to which Rihanna - born Robyn Rihanna Fenty - replied "Thanks!"  It was the first time in a long time that the former flames had communicated directly with each other on Twitter.  Earlier this year a judge eased a restraining order, allowing the pair to have contact with each other.  Brown  remains on supervised probation, however, after pleading guilty in 2009 to assaulting Rihanna, his then-girlfriend.

Though there've been rumors that Brown and Rihanna have rekindled their romance, Brown's rep has said that is in a relationship and not cheating on his girlfriend.  One listen to Rihanna and Brown's verses on the highly suggestive "Birthday Cake" remix though, and speculation about whether the former pair are dating again is likely only to grow.