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All year long, Hollywood watchers have wondered which starlet would snag newly single Ryan Reynolds. Mystery solved!

A source tells the new issue of Us Weekly (out Wednesday) that the Green Lantern star, 34, and Charlize Theron, 35, "have been dating -- for months! They're exclusive, and it's very hush-hush."

While the two haven't been photographed together, a witness noticed Reynolds' motorcycle at Theron's home all morning on June 5, then saw the actor exit her house around 3 p.m. -- and she left just minutes later.

Has Reynolds (who split from wife Scarlett Johansson last December) found a perfect match?

"They're both career-focused," explains the source, "but not in a crazy way."

For instance, don't expect to see the stars side-by-side on a red carpet. "She won't go to an event with him," the insider says of Theron, who split with love of nine years Stuart Townsend in 2010. "That's not her style."

Meanwhile, Reynolds hasn't completely shut off contact with famous ex Johansson, 26. He and the Iron Man 2 actress were spotted having a cordial dinner last Friday at Little Dom's in L.A., an observer confirms to Us.

Tell Us: Do Reynolds and Theron make a cute couple?