Katy Perry and Britney Spears revealed that they had secretly worked on a song featured in the film, when they attended the premiere of 'Smurfs 2' in Westwood, California.
Perry told E! News, with regard to Spears' song 'Ooh La La' from the animated movie, that she and Bonnie McKee recorded some background vocals for the track, in secret, Us magazine reported.
Perry added that cinemagoers should listen to the song, and make out who's who.