Jennifer Morrison Dating Once Upon a Time Costar Sebastian Stan

There may be a happily ever after for two stars of Once Upon a Time.

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.”

Caught in the Act!

• Just call it a Gossip guys’ night out: Ed Westwick, Chace Crawford and Sebastian Stan partied together at the Hiro Ballroom inside New York’s Maritime Hotel. The guys spent the night downing Don Julio tequila and talking with costar Kelly Rutherford, who was ready to party. “Kelly was there in full-cougar mode,” a source says. “She was jokingly flirting with Chace.”

Caught in the Act!

• Leighton Meester and boyfriend Sebastian Stan, sharing champagne during a brunch party at Day & Night Restaurant & Beach Club in Southampton, N.Y. The two were partying poolside – in a cabana – alongside Mischa Barton.