Lady Gaga and Cher's 'Greatest Thing' "became a party" when they decided to make it a duet.
The producer behind the track - regular Gaga collaborator RedOne - said the song was something they had started work on together, but remained unfinished until he played it to Cher.
He told MTV: "Gaga and I recorded the song a few years ago, and we were always in the studio enjoying that song. It's weird we never did anything with it.
"So I remember my management and Cher's management were trying to get us together, and I thought, 'Oh my God, I'd love to meet Cher, just to meet her!' And so I met her, I played her some songs, and I played her 'The Greatest Thing,' and she loved it. She said, 'This is my song,' I said, 'Yes, it's your song.'
Originally Gaga was happy for Cher to record the song and to appear on the remix, but with RedOne's encouragement it became a duet.
He added:"I cut the vocals with Cher first, and I was in Madrid and got a text from Gaga saying, 'I'm thinking about jumping on the song in the remix,' " .
"I was like, 'No, you should jump on the original! Make it a duet.' She said, 'Oh yes, OK, absolutely,' and then I spoke to Cher, and it became a party."
The track will be the first single from Cher's forthcoming 26th studio album.