Fans believe this photo of Lady Gaga and Bradley Cooper proves they're having an affair ...

Their chemistry is undeniable ... so it's no surprise there's speculation Lady Gaga and Bradley Cooper are having an affair.

And the two A Star Is Born co-stars further fueled the fire with their steamy performance of 'Shallow' at the 2019 Academy Awards.

But, while Gaga is denying the rumours, fans think they've just found proof the romance is real.

A throwback photo from the Hollywood premiere for the Oscar-winning movie - which was taken in September - has been unearthed, showing what appears to be a smudge of Gaga's red lipstick on the 44-year-old actor's bottom lip.

Sharing a shot of the pair's faces, alongside a zoomed-in version that reveals the lipstick mark in even more detail, the @ladyfckinggaga Instagram account captioned the post: "Oops."

Obviously, this doesn't prove for sure if the pair are having an illicit romance.

It's entirely possible the pair went in for a kiss on the cheek and missed.

READ MORE:
• Six things you never knew about 'A Star Is Born’ ...
• Lady Gaga surprises audience by bringing Bradley Cooper on stage to sing 'Shallow' live in Vegas!

But fans still can't help but speculate, especially given the fact Cooper's partner, Irina, was walking the runway at Milan's Fashion Week on the same day.

Speaking on Jimmy Kimmel Live last week, the 32-year-old singer revealed their direction for the Oscars performance.

"People saw love, and guess what? That’s what we wanted you to see."

"This is a love song ... The movie A Star Is Born is a love story … We worked all week on that performance."

"He laid it all out. Everything that you saw — the way that it was shot, the way that they pushed the piano out [and] you saw them put the piano together, us walk up on stage, no intro — that was all him."

"It was so important to both of us that we were connected the entire time… When you’re singing love songs, that’s what you want people to feel. I’m an artist, I guess we did a good job, and, fooled ya!"

And it seems that's exactly what they did!