Jennifer Morrison Dating Once Upon a Time Costar Sebastian Stan
07/17/2012 AT 05:00 PM EDT
Following in the footsteps of Ginnifer Goodwin and Josh Dallas – whose relationship became public in April – fellow costars Jennifer Morrison, 33, and Sebastian Stan, 28, are an item, too.
"They met on set and are now exclusive," a source tells PEOPLE. "They started dating three months ago. They seem to be really happy together, but are trying to take things slow."
This isn't the first time either half of the pair has gotten cozy with a costar.
Stan formerly dated Leighton Meester when they were both on Gossip Girl, and Morrison was engaged to her former House M.D. castmate Jesse Spencer.
Stan, who has only been in three episodes of Once Upon of Time, also stars in the USA miniseries Political Animals, alongside Sigourney Weaver.