'You're So Vain' duet between Taylor Swift and Carly Simon delights fans at Swift's second sold out performance in Massachusetts. The 'We Are Never Getting Back Together' singer was joined on stage by one of her heroes, Simon, during her second sold-out performance at the Gillette Stadium in Foxborough, Massachusetts, to give fans a concert experience to remember.

Taylor Swift
Swift couldn't wait to unveil her special guest on Saturday night

The two performed Simon's hit 'You're So Vain' for the packed out crowd, who lapped up ever second of the once in a lifetime occasion. During Saturday's (27 July) concert at the 55,000 capacity stadium, Swift introduced Simon towards the end of her set to perform the iconic track, one that has no doubt influenced Swift's own songwriting (if you haven't heard the song, it's about a break-up). Clearly excited to unleash her special guest to the unsuspecting audience, Swift told the crowd before the duet; "Before I bring out my special guest, I have this question that I always, always had, and I think of you you have probably had this question, too, and wondered what the answer is. Who is the song 'You're So Vain' by Carly Simon written about?"

Although the crowd we're left to ponder that riddle, Simon apparently told the closely held secret to Swift, but unfortunately she only revealed the secret in complete confidence and Swift has been sworn to secrecy. Since the song was released 41 years ago, theories about the potential egomaniac have been rife, with Warren Beatty's name usually one of the prime candidates, although Mick Jagger, Kris Kristofferson, Cat Stevens and Simon's former husband James Taylor have each been singled out as potential targets. We wonder who Taylor was aiming the song at when she sang it...

Carly Simon
Carly Simon told Taylor who her 'vain' former love was