During Gaga's Las Vegas residency concert at the MGM Grand, titled Enigma, the singer invited Cooper, her co-star and director in A Star is Born, to join her on stage, according to E!
Gaga joked that she got Bradley in-ear headphones, and he responded that "Jackson never used these!"
Gaga then proceeded to sit at the piano to play the keys.
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On what's been a wonderful comeback after being the victim of a knife attack two years ago, she added: 'It's hurting a lot today. We learned the Japanese star she has the head and heart of a champion.
The award-winning singer brought her "A Star is Born" costar Bradley Cooper onstage to sing the film's song "Shallow" Saturday night.
Videos of the performance posted on social media show Cooper sitting on a stool to begin the song, with Gaga squatting next to him, looking up to him and nodding in encouragement as he sang. He then joined her on the piano bench and wrapped an arm around her to finish out the final chorus together.