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"A little ditty 'bout John and Ryan…"

Rocker John "Cougar" Mellencamp and actress Meg Ryan, 49, are collaborating…on a romance.

"They are indeed dating," a source tells the new issue of Us Weekly (on stands Wednesday). According to the insider, Mellencamp and the Sleepless in Seattle star were first spotted together in NYC in October.

Though Mellencamp, 59, recently called it quits with Elaine Irwin Mellencamp, his wife of 18 years, a source tells Us that "Meg came into the picture much later -- well after they decided to split."

For more on the couple's fledgling romance -- including how Ryan supports Mellencamp on the road -- pick up the new issue of Us Weekly, on stands Wednesday!