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On a quiet Friday morning in a dressing room at Madison Square Garden, the Jonas Brothers hold out their hands to show off their purity rings. Kevin, Joe, and Nick Jonas—the teen-pop trio who stand, at this very moment, on the brink of hugeness—wear the metal bands on their fingers to symbolize, as Joe puts it, "promises to ourselves and to God that we'll stay pure till marriage." Joe is 18. His ring is silver and adorned with a cross. "It actually ripped apart a little bit, just on the bottom, here, but I didn't want to get a new one, because this one means so much to me," he says. Nick, who is 15, says, "I got mine made at Disney World. It's pretty awesome." Kevin, at 20, is the oldest of the three, and while a punk-rock purity ring from Tiffany might represent the ultimate oxymoron, that's exactly what he's going for. His silver vow of abstinence is covered with studs. "It's pretty rock and roll," Kevin says. "It's getting banged up a little bit because of the guitar."

Tonight is Jingle Ball, the Z100 holiday concert that has, over the years, turned into a coronation ceremony for the new superstars of teen America. The brothers grew up in New Jersey, and just a few months ago they were still the kind of guys who would tenaciously call the radio station, trying to win tickets to the Ball. Now they're headlining. By the middle of 2008, the Jonas Brothers just might be the biggest teen band in the country. What's making them massive is not just their skill with sugar-dusted, girl-crazy gobs of pop. No, the Jonas Brothers are bound for bigness because, like Britney Spears and the New Kids on the Block and David Cassidy before them, they have been handpicked to summon all the desire, frustration, and spending power of the great American teen.

Within a few hours the dressing room starts to feel like a bunker. By nightfall, the narrow passageway leading to the room is clogged with chattering, unsupervised clusters of teenage girls in braces and low-slung jeans and UGG boots. Big Rob, the brothers' 395-pound bodyguard, glowers and shoos the girls away, but within minutes they're back, craning their necks, arching their backs, oh my God—ing into their cell phones, glossing their lips, batting their eyelashes, and fidgeting with their all-access passes. Now and then the door opens for a fleeting moment—when the film heavyweight Harvey Weinstein comes by with his daughter and a phalanx of her friends, and when Timbaland, decked out in a fur coat and a crucifix, pays an imperial visit. Each time the door slides open a couple of centimeters, the girls out in the hallway totally lose it and burst into gusts of shrieking.