The actors made their red carpet debut as a couple at The Girl On The Train premiere in New York City on Tuesday, when the 36-year-old Orange is the New Black star was spotted with a diamond ring on her left ring finger, and sources now reveal it wasn't just for show.
Insiders tell People magazine the pair is planning a wedding after a whirlwind romance.
Prepon was first rumoured to be dating the actor in July, 2016 after they were spotted together on a number of occasions, but they have never spoken about their relationship.
Ben, 35, ended his engagement to Sean Penn's ex-wife Robin Wright for a second time last year. They began dating in 2012 and he proposed in December, 2013, but they split almost a year later. They reconciled and she put her engagement ring back on, but the former couple finally called it quits last summer.
Earlier this year, he was romantically linked to rocker Jack White's ex-wife Karen Elson.
Laura reportedly hasn't been in a serious relationship since she broke up with Christopher Masterson, the brother of her That 70's Show co-star Danny Masterson, in 2007. She also dated actor Scott Michael Foster.
Prepon opened up about the difficulties of finding love as a working actress in a 2014 Cosmopolitan interview.
"When I'm working, it's next to impossible," she said. "I like to think I'll just be walking down the street one day and stop and meet someone, like, 'Oh my God, you're awesome,' and then we start dating. But, no. I do not know what's happening... and I'm totally fine - because guys occupy a lot of attention space in your mind.
"When I'm working, living in a new place, it's kind of cool I don't have to deal with that right now. That being said, it'd be cool to meet a cool New Yorker. But that hasn't happened yet."
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