Leo who?

Just weeks after her split from Leonardo DiCaprio, Blake Lively and her Green Lantern costar have been romancing each other in Boston and NYC. One witness spotted Reynolds (he officially divorced Scarlett Johansson in July) having an "affectionate" goodbye with Lively, 24, at 5 a.m. at Boston's South Station. (Reynolds, 33, is currently shooting action film R.I.P.D. in Beantown.)

"They're seeing each other casually," a Lively source tells Us of the costars. "Now they're both single and trying it out."

Get more juicy details on Lively's hot new rebound in this week's Hot Stuff video, as explained by Us Weekly Senior Editor Ian Drew -- and special guest Hulk Hogan!

Also in the roundup: Prince Harry's wild partying in the U.S., Brad Pitt's World War Z gun scare and much more.

Watch the clip now!