Hugh Hefner and Crystal Harris back together: Which on-and-off celeb couple can't stay apart?

Say it isn't so! One year after they called off their wedding just days before it was scheduled to take place, Hugh Hefner, 86, and Crystal Harris, 26, are back together.

"Yes @hughhefner and I are back together. Yes I am his #1 girl again. Yes we are happy. Hope that clears up any confusion! xo," the Playboy cover girl tweeted.

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But don't expect them to rush down the aisle. "Hef and Crystal are taking it slowly this time," a source told "They’re just getting back to their old groove together, and so far they haven't talked about marriage or a wedding."

Harris walked out on the 60-years-older Hef just days before their planned wedding last summer. "Crystal begged Hef to let her come back," a source told Radar. "He surprisingly doesn't have any hard feelings against her, so he let her move back into the Mansion."

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From Hef and Harris to Justin Timberlake and Jessica Biel to Miley Cyrus and Liam Hemsworth, let us know: Which celeb couple just can't stay apart?