Jay Z & Beyonce -- "'03 Bonnie & Clyde"

When they're feeling extra lovey-dovey, celebrity couples hit the recording studio together. It's like a rite of passage for Hollywood's hottest musical pairs. But exactly how romantic are these infatuation-driven duets? From Jay Z and Beyonce to Sonny and Cher, we put 15 songs to the test, assigning each a "heart beats" rating that ranks romance factor on a scale of 1 to 10. Usher in Valentine's Day with these love songs, and chime in with your favorite celebrity-couple duets below.