If the rumors are true, Wilmer Valderrama and Demi Lovato are back on again. And while they've both been pretty secretive about it, he apparently couldn't let the occasion of her 21st birthday pass without some sort of public acknowledgment.

On Tuesday, Aug. 20, Valderrama tweeted:

The replies from other Twitter users were quick to point out he'd called her "my Demi" when shouting her out. (Apparently they even have their own celebrity couple mashup name: Dilmer.)

Wilmer, 33, and Demi have been off and on since first hooking up in 2010, not long after her split from Joe Jonas. In the years since, Valderrama's been publicly linked to Minka Kelly, while Lovato was reportedly seeing One Direction's Niall Horan. But back in February of this year, sources said the off-again couple was very much on-again.

That same month, the 'X Factor' judge did an interview in which she waxed philosophical about romance.

"I'm in a place where I can finally trust and love and be a better person for the man I am in a relationship with, instead of being a clingy girlfriend or getting jealous," she told Cosmo on Campus (quotes via MTV). "It is very unhealthy when girls devote all of their time to a guy and forget their friends and family. I did that. I was always looking for distractions because I was so afraid of being alone."

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