Bradley Cooper joins Lady Gaga for surprise 'Shallow' duet at her Las Vegas show

Bradley Cooper and Lady Gaga reunited onstage Saturday night and it wasn't at an awards show. But it might very well be a rehearsal for one: The Oscars.

Cooper joined his "A Star Is Born" co-star at her Las Vegas residency for a duet on "Shallow," which is up for best original song at the Academy Awards on Feb. 24.

But first, he needed Gaga to help him get his earpiece squared away.

"Jackson never used these," Cooper joked to the audience as Gaga got him wired, referring to his grizzled rocker character. "I can't hear anything."

Once Gaga and the sound crew got him situated, he began the first verse as she knelt next to him. Later, he leaned over the piano as she belted out the chorus.

Related:Why was Bradley Cooper's mom and not Irina Shayk with him at the 2019 SAG Awards?

Cooper still looked like he was in Jackson mode but Gaga, dressed in little more than a blue wig, T-shirt, tights and boots, was definitely not Ally anymore.

'A Star Is Born': Good luck getting Gaga, Bradley Cooper's 'Shallow' out of your head