John Mayer Helps Katy Perry Celebrate Her 28th Birthday at L.A. Costume Party

Katy Perry's 28th birthday isn't until October 25, but the superstar kicked off her festivities early with a wild costume bash on Friday.

Dressing as a sexy vampire, Perry was joined at Los Angeles venue Magic Castle by her on-again beau John Mayer, Kristen Stewart, songwriter Diane Warren, Neil Patrick Harris and America's Next Top Model's Johnny Wujek, among other celebs.

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Though his lady was the evening's woman of honor, Mayer, 35, attended the bash with quite a head-turning get-up. To complete his vampire hunter ensemble, the musician donned a prosthetic nose and chin, according to JustJared.com.

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During the bash, Perry -- who asked for donations to support the David Lynch Foundation for Transcendental Meditation in lieu of gifts -- only had eyes for on-again beau Mayer. "They were in their own world. She didn't even talk to her friends much," a partygoer tells Us Weekly. "It was just her and John. She's obsessed with him. She said hello to people but then went right back to him."

The on-again couple also feted Mayer's 35th birthday on Tuesday, with a low-key meal in New York City at downtown eatery Freeman's.

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Rekindling their romance, Perry "believes she can change" Mayer, a source tells Us. "She doesn't want to be just another one of his conquests."