Taylor Swift teamed up with Carly Simon during her concert in Massachusetts Sunday, where Swift and Simon sang a duet of Simon’s 1972 hit “You’re So Vain.”

Swift, who is notorious for singing about her broken heart, proudly announced her special guest, but before Simon walked the stage Swift asked according to Entertainment Wise, “Before I bring out my special guest I have this question that I've always, always had. “I think a lot of you probably have this question too, and always wondered what the answer is. Who is the song 'You're So Vain' by Carly Simon written about?”

Simon smiled as she graced the stage and remained ever tight-lipped who the hit breakup song is about. As EW notes, speculation from Mick Jagger to Cat Steven’s have surfaced, but the 68-year-old singer would neither confirm nor deny speculation.

Reports claimed that Swift opted to sing the duet after allegedly being cut out of ex-boyfriend Harry Style’s One Direction movie, This is Us. However, This is Us director Henry Spurlock cleared up the rumor during an interview with Teen Vogue saying, “It gets frustrating for them at times, because suddenly stuff gets printed that isn't even true. I saw an article that said, 'Harry Styles demands Taylor Swift be taken out of the movie.' But by the time we started filming in January, they had already split up."

Swift never addressed the rumors that she sang the song as a dig to Styles, but rather gushed about the opportunity to sing alongside Simon via Twitter, writing, “Got to sing 'You're So Vain' with Carly Simon at Gillette Stadium tonight!!!!! Unreal. I could not possibly look up to her more.”’

Image: Video Screenshot