Seemingly reluctantly, the actor joined her onstage, following her to the piano before quipping that "Jackson Maine would never wear these", as he struggled with some in-ear monitors.
A Star Is Born tells the tragic love story of ME, played by Cooper, who helps Gaga's struggling artist Ally.
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Things escalated to an even more magical level when Lady Gaga took the helm and sang her part.
Sharing a picture of the performance on Twitter, one fan wrote: "As if Lady Gaga's spellbinding 2 hour greatest hits chronicling Enigma wasn't epic enough, the encore involved her pulling Bradley Cooper out of the audience to duet on Shallow".
If you attend a Lady Gaga performance, you should no doubt be ready for uninmagineable surprises. The song is also slated to be performed at the Oscars in February.
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Some of them may be later this week, but we hope that by the end of this week all of the back pay will be we'll be made up, ' he said on CBS News.
Video footage from the concert shows Gaga helping Cooper get ready to sing, giving him a kiss on the cheek before he sat down.
Gaga usually performs her award-winning song "Shallow", which is played three times in the course of A Star is Born, as the encore at the Enigma show.
The iconic song from the movie has already won the pair a Golden Globe for Best Original Song.
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