The two have actually been seeing each other since December, sources tell the London Sun.
As RadarOnline.com previously reported, Rihanna was spotted leaving the Two and a Half Men star's home early Thursday morning, four hours after she arrived.
The two were introduced at a mutual friend's party, the source tells the Sun. "The flirting began as soon as Rihanna and Ashton met and swapped numbers. That moved on to texts and arranging to meet. They snuck off to a Santa Monica hotel a few weeks ago.
Demi Moore announced her split from Ashton last November following revelations he'd cheated.
Rihanna is again working with her former boyfriend Chris Brown, apparently putting aside his violent attack on her from two years ago.