Katy Perry sings back-up for Britney Spears
Britney Spears and Katy Perry collaborated on Ooh La La! for the Smurfs movie.
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.
Katy declined to elaborate further when asked if fans could expect another collaboration.
She did say she was looking forward to being introduced the Toxic singer, who brought along sons Jaden and Sean.
"I can't wait to meet her," Katy, who took her grandmother as her date, gushed.
Britney is meanwhile working on her eighth studio album with Black Eyed Peas producer will.i.am.
The 31-year-old pop star said last month she thinks people will be "surprised" by the musical approach and she's loving the experimental process.
"Well it's definitely the beginning stages right now. I want to take this album and have a new approach with it and make it different from anything I've ever done," she explained during an On Air with Ryan Seacrest interview.
"There's a lot of guitar and I think people will be surprised with this album. I think the kids will appreciate it but I think it will have a cool approach to it. [It should be out] by the end of the year," she said.
© Cover Media