Britney Spears + Katy Perry Collaborate on Song, Hang at ‘Smurfs 2′ Premiere [Pictures]

Britney Spears and Katy Perry are forever linked by 'The Smurfs 2' -- and not just because Brit Brit sings 'Ooh La La' (the ending credits song) and Perry voices Smurfette in the flick. Perry actually sang backup on 'Ooh La La,' too! True story.

The song was penned by Bonnie McKee, who is working with Perry on her follow-up to 'Teenage Dream,' and who is making a go of it as a solo artist in her own right! And that's how Perry's voice ended up as that of a background vocalist.

"Me and Bonnie secretly did some background vocals on that song," Perry told E! at the film's premiere this weekend. "So maybe listen for our voices."

Um, you bet we will. We will tune our sonar for Perry's vox in the song!

Brit Brit and Perry met up at the premiere, both rocking blue dresses in different styles and hues. They both looked like the hot pop divas they are, though.

Perry wore a prim and proper Tadashi Shoji dress in a cobalt-navy shade. It got its edge from the leather skirt.

She also brought her grandmother Ann Hudson along for the ride. How CUTE! Check out Gram's kitschy shirt and her blinged out cane. We so want to hang out with that sassy dame. They don't make 'em like they used to.

Perry has really nice teeth, doesn't she? She looks thrilled to be fraternizing at the film's bow.

Check her out as she hangs with Smurfette. Hey, sexy ladies!

While Perry wore a classic, tea length frock, Brit, on the other hand, went dangerously short and tight, donning a turquoise Herve Leger bandage dress that is pretty much painted on like a second skin. She looked HAWT. It also had some black piping. The heels? Those are Saint Laurent.

Perry wasn't the only one to bring her family to the premiere, either. Brit Brit was on hand with her boys: Sean Preston and Jayden James. They are adorbs and have cameos in the "Ooh La La" vid.

Brit in mommy mode -- is there anything cuter?

Brit's kids are freakin' presh. They are going to grow up to be heartbreakers...just like mom.

Brit also shared some pictures on her Instagram.