Who knew Britney Spears and Katy Perry have made music together? It turns out Perry actually sings on Spears.The catchy pop tune, written by frequent Perry collaborator Bonnie McKee, is played over the end credits of the Smurfs 2 movie. Selena Gomez's 21st birthday celebrations continued this weekend with a gypsy-themed party and an on-again, off-again flame. Justin Bieber, Derek Hough and Jaden Smith were just some of the famous young faces spotted heading into Gomez's lavish bash in Malibu, Calif. on Saturday night. Nearby photographers caught the Biebs holding a single rose (perhaps for the birthday girl?) after arriving to the party in his leopard print Audi. Looks like all is back to normal for Emma Roberts and Evan Peters. The pair were spotted on the set of "American Horror Story" looking cozy. This, after Roberts found herself arrested by Canadian officials after getting into a bloody brawl with Peters. It looks like there are no hard feelings here.