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The No Strings Attached actor was back at her side though, the following day – when the pair were spotted leaving the New York hotel where Demi was staying.
Ashton looked a little tense, given the throng of photographers vying to get a snap of them.
Meanwhile his wife hid behind him, holding onto his arm.
They must have known that Demi's solo appearance at the fashion gala would put the spotlight onto their romance.
But the couple are used to speculation over their relationship, and clearly don't let it bother them.
The last time they were photographed together, they were putting their energies into a joint crusade - a campaign to end child sex slavery.
Looking still very much united, the couple posed on the red carpet at a New York launch for an internet campaign for their cause.
Certainly, if Demi is under any stress, she wasn't showing it as she attended the Costume Institute Ball.
Gorgeous in a Prabal Gurung black embroidered tulle strapless gown with ostrich feathers, Demi looked excited and happy to be at the event.
And though Ashton wasn't around, the 48-year-old was joined by her actress daughter Rumer – and one other very special guest.
"Look who is going to the Met Ball!" she tweeted, before posting a picture of herself in her outfit with Purse Monkey – a little pink monkey toy she sometimes takes to red carpet events.