Katy Perry parties with Justin Bieber

Katy Perry partied with Robert Pattinson, Justin Bieber and Selena Gomez after the premiere of her new movie.

After attending the screening of 'Katy Perry: Part of Me 3D' in Hollywood, the group met up at the infamous Chateau Marmont hotel on Tuesday (26.06.12).

The group then travelled in Justin's $200,000 Mercedes Benz minivan to celebrate the opening of David Arquette's new nightclub, Bootsy Bellows.

Despite 18-year-old Justin and 19-year-old Selena being under the legal drinking age in America, the couple enjoyed the evening out with 26-year-old Robert and 27-year-old Katy.

A source told UsMagazine.com: "Rob and Katy are good friends so he was happy to celebrate with her, it wasn't just the four of them, but everyone was close."

Apparently 'Twilight' actor Robert has been keeping in touch with pop star Katy after her divorce from Russell Brand last December.

The insider said: "He's just checking in on her. It's not romantic at all. They're friends."

His girlfriend and co-star in the vampire movies, Kristen Stewart, was unable to join him as she was attending a Balenciaga event in Paris, France and has been busy promoting 'Snow White and the Huntsman'.