A friend of Christenson and Bilson says the pair became engaged over the holidays.
"They're so excited," says the insider, who says no wedding date has been set.
"They're a great couple. Rachel seems thrilled beyond belief."
Another spy overheard a conversation on a recent flight between Bilson and her seatmate. "A man sitting in front of her on the plane saw a ring on her finger and asked if she was married," said the snitch.
"'I'm not married yet, just engaged,'" was Bilson's reply to the man. "'I know I look like I'm 15, but I'm actually 27.'"
Bilson and Christensen met on the set of the 2008 film Jumper. The bomb at the box office ignited fireworks off-screen.
But the couple is remaining mum. "I don't comment on my clients' personal lives," says Bilson's rep. Christensen's had no comment at all.
Bilson previously dated Adam Brody, her costar on The OC. Christensen rose to fame in the Star Wars prequel trilogy as a young Darth Vader.
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