Katy Perry turns 28 on October 25, but she kicked off her birthday celebration with a wild costume party on Friday. The singer was joined at the Magic Castle in Los Angeles by on-again, off-again boyfriend John Mayer , Kristen Stewart , Neil Patrick Harris and songwriter Diane Warren .
Perry was dressed as a sexy vampire, while Mayer showed up as a vampire hunter, complete with a prosthetic nose and chin, according to photos posted on JustJared.com .
A party insider tells Us , Mayer and Perry only had eyes for each other and she “didn’t even talk to her friends much.” The partygoer insists, "It was just her and John. She’s obsessed with him. She said hello to people but then went right back to him.”
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