Home alone -- and abroad -- with Macauley Culkin's little bro?

Although Scarlett Johansson had a super-flirty all-nighter with Justin Timberlake Sept. 14, he's not the only Hollywood guy she's hanging out with.

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On Sept. 12, Johansson, 26, was photographed strolling in Paris with actor Kieran Culkin. Five days later, she and Culkin, 28, were seen strolling arm-in-arm outside Manhattan lounge Cabin Down Below.

But a source tells Us Weekly that Johansson and the Scott Pilgrim vs. the World star, 28, are just old friends who went to high school together.

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"She has never hooked up with Kieran," the source insists.

After she and Ryan Reynolds finalized their divorce earlier this year, Johansson dove into a relationship with Sean Penn; the 50-year-old Oscar-winning actor ended the romance after five months in late May.

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She and Timberlake, meanwhile, have "hooked up several times over the years," a pal explains. But despite their chemistry, "they've always been fine keeping it just sex and friendship."

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