Rihanna and DiCaprio 'keeping things private'

Leonardo DiCaprio "makes Rihanna's knees tremble".

The pair were first linked after they were reportedly seen kissing during a party at the Playboy mansion last weekend.

And while the occasion may have been the first time Leo and Rihanna have been seen together, they have allegedly been hooking up for several months.

"They mostly get together during the middle of the night. She creeps out late at night to get it on — it's like a drug," a source told HollywoodLife.com. "It's a whole process.

"She switches cars and drivers and meets him in very discreet places. They've got a system and they're keeping it between themselves and extremely private."

Rihanna has been single since splitting from on/off boyfriend Chris Brown, while Leonardo ended his romance with model Toni Garnn in December.

And despite the fact that Leonardo and Rihanna's dalliances are purely casual, things are apparently going well for the new couple.

"Rihanna's smart about it too," the source added. "She's not blowing him up. She's not trying to chain him down either. It is what it is."

Leonardo has been linked to a string of leggy blonde models over the past few years. But it seems he may have found his perfect woman in Rihanna – and the feeling is very much mutual.

"Leonardo's funny. He's real, honest and authentic," the source concluded. "He holds a good conversation but the one thing that makes her weak every time she looks at him are his eyes. That alone makes her knees tremble."