Chris Brown and Rihanna collaborate on two song remixes

(CBS/AP) NEW YORK - Chris Brown and Rihanna are back together - musically speaking.

Two songs featuring the singers hit the Internet on Monday. Brown sings on the remix of Rihanna's explicit song "Birthday Cake," and she's featured on the remix of his tune "Turn Up the Music."

Pictures: Chris Brown
Pictures: Rihanna

Brown shared a link to the new version of "Turn Up the Music" via Twitter on Monday, and Rihanna later retweeted it. She also tweeted about the remix for "Birthday Cake."

Brown attacked his then-girlfriend Rihanna at a pre-Grammy Awards party in 2009. He pleaded guilty to a felony charge and is serving five years of probation.

Last year, a judge eased a restraining order after an attorney for Rihanna said she didn't object to removing the stay-away provisions. The order had required Brown to stay 50 yards away from Rihanna, but the restriction was reduced to 10 yards if they were at a music industry event.

After a three-year absence from the Grammys, Brown made his return this year, performing twice at the show and winning the award for best R&B album. Rihanna also performed and picked up a trophy.

Rihanna told Esquire magazine in its October 2010 issue that she no longer resented Brown and that she enjoys his music.

The two singers first collaborated musically in 2007 on the remix to her monster hit "Umbrella."

Brown also took to Twitter to wish Rihanna (whose real name is Robyn Rihanna Fenty) a happy birthday. She turned 24 on Monday.

A representative for Rihanna said she had no comment. An email seeking comment from Brown's representatives went unreturned Monday.