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It was a Gossip Girl Halloween miracle when Leighton Meester (hiding behind a huge flower in these photos) took to the stage to help Weezer sing their new single, “(If You’re Wondering If I Want You To) I Want You To.” Watch the video by clicking the continue reading link below.

At a Halloween performance at the Hammerstein Ballroom in New York, aspiring pop star Meester joined lead singer, Rivers Cuomo in a duet. The band’s frontman explained to MTV News how the collaboration came to fruition.

“We invited her down, and she was in rehearsal–wait, did she even come to rehearsal? No, she didn’t!” Cuomo claimed. He added, “We just worked on it in the dressing room right before the show. ‘You sing this line, now I’ll sing that line,’ just like that. It’s best when there’s a little rehearsal, so that it can still feel spontaneous.”

Spontaneous or not, it’s a brilliant cross-promotional move on both their parts. Well-played, kids.

Gallery Info: Leighton Meester at the 2009 Library Lions Benefit in New York on November 2, 2009.