Bradley Cooper joined Lady Gaga on stage Saturday night for a surprise performance of their hit song, "Shallow."

The actor appeared alongside his A Star is Born co-star at her Las Vegas residency for a duet of the popular song. Gaga, 32, helped fit Cooper with an earpiece before they started singing together.

"Jackson never used these," he joked, referencing his character in the film. "I can't hear anything."

As soon as he was wired, Cooper led the performance. Gaga knelt beside him for the first portion of the performance. She then performed the chorus while he leaned over the piano on stage.

Cooper, 44, sporting his long locks and casual clothes bore a resemblance to his A Star is Born persona, while Gaga switched things up completely. The musician took the stage in a blue wig, T-shirt and tights for the show.

Fans were blown away by the impromptu duet. Many flocked to Twitter to fawn over the pair and crack a few jokes.

It's unclear if Cooper will join Gaga at future performances.