has been busy celebrating her 24th birthday in London, and guess who took the time to wish her a very happy birthday? None other than super ex-boyfriend .

After dodging months of reunion rumors, it looks like Chris and RiRi are now publicly tweeting one another.

Brown — once convicted of battering the “Rude Boy” songstress — tweeted a message to Rihanna, who celebrated her birthday on Monday.

“HAPPY BIRTHDAY ROBYN!” he wrote, using the singer’s real first name.

“Thanks!” she tweeted back.

The friendly banter doesn’t stop on the social network. RiRi also enlisted Brown to appear on her upcoming single “Birthday Cake,” a collaboration that the song’s producers, Da Internz, said would “shock the world.”

The pair previously teamed up on a remix of Rihanna’s Grammy-winning 2007 single “Umbrella.”