Robert Pattinson, Katy Perry Party in Minivan With Justin Bieber, Selena Gomez

Justin Bieber and Katy Perry arriving at the opening of "Boosty Bellows" Night Club on Sunset Blvd in West Hollywood, CA on June 26, 2012. SPW/Splash News Online

Random party pals!

In a very unlikely pairing, Robert Pattinson, Katy Perry, Justin Bieber and his girlfriend Selena Gomez partied in Hollywood together Tuesday night.

The foursome met up at L.A. hotspot Chateau Marmont Tuesday before they hopped into Bieber's $200,000 Mercedes Benz minivan and traveled together to the opening of David Arquette's new Hollywood club, Bootsy Bellows. Despite Bieber being only 18 and Gomez 19, the young stars partied with their older pals Perry, 27, and Pattinson. 26.

A source tells Us Weekly that Perry threw a small party to celebrate her new film, Katy Perry: Part of Me 3D before heading to the club opening. And no, it wasn't a double date. "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," the source explains to Us.

robert pattinson
Robert Pattinson arriving at the opening of "Boosty Bellows" Night Club on Sunset Blvd in West Hollywood, CA on June 26, 2012. SPW/Splash News Online

Although the Twilight star is attached to his costar Kristen Stewart, Pattinson has been supporting his friend Perry by keeping in touch with calls and text message after her divorce from Russell Brand last December. "He's just checking in on her," a source told Us. "It's not romantic at all. They're friends."

Perry, who's currently sporting purple highlights, wore a sexy strapless black dress for the party. Pattinson kept it casual with a baseball cap and jeans.

And "Boyfriend" singer Bieber, who provided the transportation for the evening, enjoyed the date night with his lady. "Justin and Selena were cute together," the source tells Us. "Katy didn't want the night to end."

Why wasn't Stewart, 22, by Pattinson's side? Unfortunately she's attending a Balenciaga event in Paris, France and has been busy promoting Snow White and the Huntsman. But the couple were recently reunited for a wedding in New Jersey on Saturday. A wedding guest told Us, "Rob and Kristen were all over each other."

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