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The couple were seen kissing and hugging at a party in LA last week, prompting rumours that they were giving their romance another try.
However, Olsen's spokesperson insisted that there was no truth in the claims, saying: "They aren't back together. They're friends."
Leto previously dated Cameron Diaz, while Olsen has been linked to Tour de France winner Lance Armstrong and actor Josh Lucas.
Ashley and twin Mary-Kate swapped their £2.5 million California home for a smaller house in LA's Larchmont Village last month.
