Chris Brown wished ex-girlfriend Rihanna a very happy birthday on Monday.

And his greeting was personal, referring to the Rihanna by her legal name, Robyn.

HAPPY BIRTHDAY ROBYN! the 22-year-old Brown wrote.

To which Rihanna, who turned 24 on Monday, responded, Thanks!

The Twitter birthday exchange is the latest to fuel rumors that the two are back together.

Just last week, after a Brown-filled Grammy weekend (that came with its own controversy), the singer was reportedly one of 100 guests invited to Rihanna's private Caribbean themed early birthday party in Beverly Hills.

According to TMZ, Brown attempted to make guests sign a confidentiality agreement. While he failed at getting everyone to sign the agreement, Brown stayed close to Rihanna, as the two were spotted talking and holding hands, sources told TMZ.

Brown is also a rumored guest on Rihanna's upcoming single, Birthday Cake. After consulting with a pair of independent sources, hip-hop radio station Hot 97 confirmed that Brown contributed a singing verse and rap verse to a remix version of Birthday Cake.

But it looks like the song might be taking a hike.

Rihanna is allegedly being pressured to shelf the 'Birthday Cake (Remix)' completely. It's possible that she recorded it with Chris without alerting label and management, Hot 97's Miss Info wrote on her blog.

The last time the pair made headlines of this nature was of course in 2009, when Brown assaulted Rihanna just one day before the 52nd Grammy Awards. Rihanna's restraining order on Brown was lifted nearly one year ago.

With rumors that Rihanna and Brown are back together, where does that leave Brown's model girlfriend Karrueche Tran?

According to sources close to the 23-year-old model, Tran is unfazed by all the noise. And Brown reportedly professes his love to Tran daily, the sources recently told TMZ.

Brown is gearing up for the release of his fifth studio album, Fortune, out in March.  

Rihanna's latest album, Talk That Talk, has sold more than 600,000 copies in the US since its November 2011 release. She will also star in the upcoming film Battleship, based on the game of the same name, out in May.

Do you think Rihanna and Chris Brown are back together? What do you think of the birthday tweets and the possibility that Brown may appear on Rihanna's next single, Birthday Cake? Share your thoughts below.