IF this is true, and it's a BIG if, talk about moving on quickly!

Newly-single Gossip Girl star Leighton Meester had at least one overnight hookup with none other than Robert Pattinson, according to new, scandalous reports.

The Twilight saga hunk was recently spotted entering Leighton’s New York apartment, an did not leave until 11 the next morning. Taking his sweet time, eh?

“He ducked out of the building and hopped into a waiting car," says a source." He apparently spent the night with Leighton, and didn’t want anyone to know!”

Adding fuel to the (red hot!) firestorm of rumors surrounding Robert Pattinson and Leighton Meester? He even carried an overnight bag during his hasty exit.

Suggesting he'd been there and done this before, perhaps?!

OMG, LOB: Leighton and Rob can't ... can they?! [Photos: PacificCoastNewsOnline.com]

A friend of Leighton supposedly "confirms" the affair, adding that the pair are still in constant contact. “She’s having a texting romance with him, and is so into it."

"She talks with Rob almost daily.”

Leighton and Sebastian Stan, her boyfriend and sometime co-star, recently broke up. Could she have already moved on and stolen Kristen Stewart's co-star love?

It's been awhile since any Robsten sightings were reported, but with their schedules, and all the rumors flying about them, it's hard to derive much from that.

We can see it happening, but need more evidence before we truly believe it.

Would you like to believe it, though? Do you think Rob and Leighton would be a hot couple? Would you prefer her with another Brit ... named Ed? Discuss!

Steve Marsi is the Managing Editor of TV Fanatic. Follow him on Google+ or email him here.