Stephanie Pratt At Holly Montag's Party Before DUI

Sources say cops were waiting outside to follow her.

The Hills stars stick together...usually. When Stephanie Pratt left Holly Montag's birthday party alone Saturday night, it's reported cops were waiting outside to follow the 23-year-old before stopping her for a DUI.

She's said to have been stopped outside L.A's Empire club with a blood-alcohol content just above the legal limit. Pratt was taken into custody at 3:45 a.m.

Doesn't sound like she'd planned to make it a late night. Steph said on Twitter earlier that evening, "It's my sissy @hollymontags bday party! Just finished dinner and going to Empire but I'm pretty tired aka yes i am the party pooper :("

Fortunately her bail of $5,000 isn't too steep for the Hills star.

Jon Gosselin was another reality tv star who enjoyed a ritzy Saturday night out. Jon lived it up with girlfriend Hailey Glassman in New York -- read about it here.

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