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"SexyBack" meets Full House!

The new Us Weekly, out Wednesday, reports that Justin Timberlake and Ashley Olsen -- both newly single, former child stars with their own fashion labels -- have been inseparable (and flirty!) in and around NYC of late.

One insider close to the pair (spotted together at the Greenwich Hotel, a Saturday Night Live afterparty, two Broadway shows and a polo match outside the city) tells Us: "They are hooking up."

Adds another source of the unexpected twosome, who both ended high-profile relationships in March (he with Jessica Biel after four years together, she with actor Justin Bartha after two years): "They're really trying to keep it on the down-low."

(A rep for Timberlake denies anything non-platonic, telling Us: "They are friends. They are not romantically involved.")

Still, an Olsen source says that the former sitcom kid turned fashion mogul is "aware of Justin's reputation with women, but...They're in the same head space."

Read more about this surprising new coupling in the latest Us Weekly, on stands now!