Pop star Katy Perry secretly recorded back-up vocals for Britney Spears' new smurfs song Ooh La La. The Firework hitmaker, who reprises her role as Smurfette in animated movie sequel Smurfs 2, has revealed she is featured on the track her frequent collaborator Bonnie McKee wrote for Britney, which plays over the end credits of the new film. Perry says, "Me and Bonnie secretly did some background vocals on that song. So maybe listen for our voices." Katie and Britney posed for pictures together on the red carpet at the weekend premiere (28Jul13) and the Toxic singer later revealed on Twitter.com it was their first time meeting - but it won't be their last. Spears wrote, "So great to see you Smurfette @katyperry! Can't wait for our Vegas date." Britney is planning a residency in Sin City and it seems Perry might be joining her onstage!