Fifty-six years ago today, on Aug. 5, 1962, Marilyn Monroe died at the age of 36 of an overdose in her Los Angeles home.

Just three months prior to her death, Monroe, dressed in a skin-tight, translucent dress, sang “Happy Birthday” to President John F. Kennedy at Madison Square Garden ten days before his 45th birthday.

That now legendary performance on May 19, 1962 led to what is believed to be the only known photograph of Monroe and Kennedy together.

Read More: What we know about John F. Kennedy and Marilyn Monroe's alleged relationship

The photograph was taken that evening at an after-party at the Manhattan townhouse of Hollywood exec Arthur Krim, by Cecil Stoughton, the official White House photographer, Time reports.

Also seen in the photo are Attorney General Robert F. Kennedy, on the left; Harry Belafonte, who also sang that night, standing in the back, and historian Arthur Schlesinger, Jr., the smiling man wearing glass on the right.

Stoughton had been instructed not photograph Monroe with the president, but he managed to sneak this shot. Out of respect for Jackie Kennedy, he kept the photograph secret.

Schlesinger later wrote: "The image of this exquisite, beguiling and desperate girl will always stay with me. I do not think I have seen anyone so beautiful; I was enchanted by her manner and her wit, at once so masked, so ingenuous and so penetrating. But one felt a terrible unreality about her - as if talking to someone under water. Bobby and I engaged in mock competition for her."  

That night's performance  added fuel to the rumors that Monroe was having affairs with both Kennedy brothers.

According to one of Monroe’s biographers, James Spada: “It was pretty clear that Marilyn had had sexual relations with both Bobby and Jack,”

Donald Spoto, another biographer, says that the actress and the president met four times between October 1961 and August 1962.

Read More: Bobby Kennedy and Marilyn Monroe affair confirmed by letter

Her masseur Ralph Roberts claimed their only “sexual encounter” took place in a bedroom at Bing Crosby’s home on March 24, 1962, just two months before Monroe’s performance at MSG.

“Marilyn gave me the impression that it was not a major event for either of them: it happened once, that weekend, and that was that,” said Roberts.

A print of the famous photo is up for auction at Lelands. It is the only surviving version of the photo that Stoughton printed himself from the original negative. ( A copy of the photo is part of the Life  Images Collection.) Bidding for the photo ends on August 17.

Cecil Stoughton's famous photograph of Marilyn Monroe, John F. Kennedy, and Robert F. Kennedy.THE LIFE IMAGES COLLECTION/GETTY IMAGES