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.
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.
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.
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."
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