Certainly friends, definitely party pals and maybe more, Heath Ledger and Mary-Kate Olsen first met over the summer of 2006, at the time he was living at the Chateau Marmont in Hollywood, PEOPLE has learned.

More recently, “Mary-Kate and Heath were casually dating for three months before Heath’s death,” a source has told PEOPLE. “They were hooking up, but neither were particularly interested in making it exclusive.”

As for what drew the two together, says the source, “They had a bond that was based on partying, and they had the same tastes in partying … like, in terms of where they liked to hang out in New York, what time they would want to go out. They just had the same sensibility.”

At the time of their first becoming acquainted – the summer after Ledger’s Oscar nomination for Brokeback Mountain – Ledger and Olsen were spotted together having lunch at the Chateau with a group of friends, and Ledger was described as being charmed by Olsen, who made him laugh as he would scarf down a big meal.

“Mary-Kate made him laugh a lot,” an eyewitness recalled.

Both the New York Post and Daily News reported this week that Ledger, 28, and the Olsen twin, 21, were dating.

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