Which Gossip Girl Star Got Hot & Heavy Last Night?

One castmember, who used to date a Gossip girl, was spotted with a brand new girl last night

Spotted: A Gossip guy getting cozy with a new actress who definitely isn't his one-time girlfriend and costar.

The dude in question did some dirty dancing at the Eldridge in NYC with Margarita Levieva (from HBO's How to Make It In America) before they left together at 2 a.m.

So who's the guy?

It's Sebastian Stan! Remember, he used to date Leighton Meester?

"They were dancing and grinding. It was pretty crazy," a source tells E! News of Sebastian and his new lady last night. "I doubt he's with Leighton after they way he was acting."

As if the girl who plays Blair Waldorf would ever tolerate that kind of behavior from a boyfriend!

Leighton and Sebastian were rumored to have split in April, but their reps never officially confirmed the breakup.

And it seems Stan may have a thing for dating costars, since he and Margarita first worked together on the Ashton Kutcher movie Spread in 2009.

So now that Leighton's single and her ex is moving on, who should she date next? We hear Prince Harry is single again if she's into the royals like her on-screen character.