Demi Lovato has hit the studio with a mystery man!

The 21-year-old singer teased fans yesterday when she tweeted a photo of her hand giving a thumbs-up next to another thumbs-up with the message that she had just finished recording a duet. But who does the second thumb belong to?

Twitter

So far there are a couple of contenders. Demi’s uppercase “KILLER” could be a hint that the hand belongs to Brandon Flowers of the Killers. Who doesn’t love a cryptic tweet with a pun, right?

Or maybe the thumb belongs to Nick Jonas! Demi and the former Jonas Brother have had a long-term working relationship, so it wouldn’t be all that surprising. Plus, fans-turned-detectives pointed out that the duet hand in the Twitter photo is wearing a gray suit and white shirt, which just happens to be the outfit that Nick had been seen sporting earlier in the day.

Does Nick's gray suit match up to the one in Demi's pic?

Rachel Murray, Getty Images

What do you think, PopCrushers — does the mystery thumb belong to Brandon, Nick or someone else entirely?