The Firework singer’s frequent collaborator Bonnie McKee wrote the track, which features in new animated comedy Smurfs 2.
The song plays over the end credits of the movie.
But cinemagoers will hear something a little different than usual, Katy revealed to E! News at the Los Angeles premiere of the film.
“Me and Bonnie secretly did some background vocals on that song. So maybe listen for our voices,” she said.
Katy and Britney didn’t meet until the Hollywood event.
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