Laura Prepon & Ben Foster Flashing Engagement Rock

Laura Prepon is weighing in a little heavier these days after Ben Foster slipped a diamond engagement ring on her ... which made its debut in NYC.

Ben and Laura have only been together, at least publicly, since the summer, but he popped the question anyway. The "Orange Is the New Black" star strategically positioned her bling Tuesday night at the premiere of her new movie, "The Girl on the Train."

It's Laura's first engagement, and Ben's second. He was engaged to Robin Wright, but they called it off in 2015.

Congrats!