Gaga was performing her 'ENIGMA' show at the Park Theater at Park MGM when Bradley appeared on stage, and people in the crowd went wild.
They sang a duet of their song "Shallow," after B.C. overcame some difficulty in installing his earpiece.
It's a super smart move ... Bradley and Gaga are best up for Oscars -- Best Actor and Best Actress -- and this type of publicity keeps them and the film front and center.
Although the film snagged 7 noms, lots of people think the Academy snubbed Bradley by overlooking him in the Best Director category.
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