Carly Simon has let Taylor Swift in on the biggest secret of her career after divulging the inspiration for her hit track You're So Vain. The young pop star brought the music veteran out on stage with her at a gig in Massachusetts on Saturday night (27Jul13) for a duet of Simon's famous song, which is long rumoured to have been written about one of the singer's famous former lovers. Possible subjects have included James Taylor, Sir Mick Jagger and music mogul David Geffen, and Simon has never divulged the answer publicly, but she has now told Swift the secret. Speaking in a joint interview with Simon backstage on her RED tour, Swift says, "I was thinking she might tell you (the secret), but I do know now." She adds of the song, "When I heard You're So Vain, I thought, 'That is the best song that has ever been written.' That is the most direct way anyone has ever addressed a break-up. It's amazing." Twift has become famous for writing songs about her past relationships without divulging the famous names who inspired her hits.