It is one of the most enduring mysteries in music: who is the subject of Carly Simon's 1972 hit
Now the singer has provoked further speculation by performing a "lost" fourth verse in which she sings: "A friend of yours revealed to me / That you'd loved me all the time / Kept it secret from your wives / You believed it was no crime."
That reference to "wives", if it is to be taken literally, rules out several contenders who were either unmarried or only on wife number one when the song came out. They include Mick Jagger, Cat Stevens, Kris Kristofferson and David Geffen, the record mogul.
Instead, it fits the description of another man: Willie Donaldson, a British Lothario who was the first man to break Simon's heart, and whom she described in her 2015 memoir as "a sardonic, adorable, long-legged, coffee-scented charmer". (It may be worth noting that coffee is mentioned in the lyrics of the song.)
To say Donaldson had a colourful life is an understatement. The son of a shipping magnate, he was educated at public school, inherited and spent a fortune, smoked crack and at one point took up residence in a London brothel. He was also the author of the Henry Root letters, a famous literary hoax that saw him write to the great and the good, from Esther Rantzen to Margaret Thatcher and the Archbishop of Canterbury, in the guise of a retired wet fish merchant.
He met Simon when he was a theatre impresario in Soho and co-producer of Beyond the Fringe. A mutual friend introduced them in 1965, when Simon was a 20-year-old ingenue on holiday in London and he was a 30-something rake.
The encounter "would prove crucial to the rest of my stay, not to mention life", she wrote in her memoir.
Six weeks into their romance, she sailed for the US, but not before Donaldson had promised marriage and introduced her to friends as "the next Mrs Donaldson".
The singer was packing her belongings in preparation for a new life in England when she received a letter from him: he was dumping her for the actress Sarah Miles.
Donaldson had married twice by the time Simon wrote You're So Vain - to Sonia Avory in 1957, and to Claire Gordon in 1968. (He would go on to marry for a third time).
He later called Simon "the answer to any sane man's prayers; funny, quick, erotic, extravagantly talented". He died in 2005.
Simon, 71, performed the forgotten verse for the first time in a BBC Four documentary, Classic Albums.
Over the years she has given conflicting clues to the man who "walked into the party / Like you were walking onto a yacht". In 2015, she said that the second verse is about Warren Beatty. "Now, that doesn't mean that the other two verses aren't also about Warren. It just means that the second one is," she teased.
Simon continues to gain publicity for her music thanks to the enigmatic man in the song. She once said of the mystery: "I could never really solve it because if I did, then no one would have anything to talk to me about."