Secret romances celebrities took to the grave – including Mary Astor's scandalous diary entries

Actress Mary Astor kept a diary documenting her thoughts on fame, her career and other – more scandalous – secrets. One in particular caused a sensation in the press: a passionate affair with married playwright George S. Kaufman.

Details in the 'Purple Diary' included secret meetings between the couple which were weaponised during her 1935 divorce and 1936 custody battle against ex-husband Dr. Franklyn Thorpe, who stole the diary and used it to argue Mary was an unfit mother.

Rock Hudson maintained a leading man reputation throughout his life, marketed as a rugged ladies' man with star roles in romantic hits like Pillow Talk and Magnificent Obsession. 

But in his posthumous biography Rock Hudson: His Story, it was revealed that Rock had considered his boyfriend of three years Lee Garlington, who he dated between 1962 and 1965, his "true love". The star publicly confirmed he was diagnosed with AIDS in 1984 and tragically died the following year

Famed actor Montgomery Clift, known best for A Place in the Sun and From Here to Eternity, kept his sexuality a secret throughout his life, though he's now known to have been gay or bisexual.

While the star was occasionally linked to stars like Mira Balaban, he was actually romantically involved with Roddy McDowall, after being introduced by Montgomery's close friend Elizabeth Taylor.

Loretta Young and a then-married Clark Gable worked together on the set of 1935 feature film The Call of the Wild, reportedly embarking on an affair which resulted in a daughter – though Loretta has since described the event as an assault.

The actress concealed her pregnancy, travelled to Europe and constructed a story of adopting the child, officially confirming the true story in her posthumous 2000 biography Forever Young. Clark, however, never acknowledge the child as his own.

The Blue Angel and Morocco actress Marlene Dietrich was one of Old Hollywood's most recognised leading ladies, carrying her star status through the '30s, '40s and '50s.

It was only recently, however, that she was revealed to have dated both men and women, with biographer Diana McLellan detailing in The Girls: Sappho Goes to Hollywood that Marlene had a secret affair with iconic actress Greta Garbo.

While the the Rebel Without a Cause actor had several high-profile romances, including a brief engagement to Italian actress Pier Angeli and a brief affair with Ursula Andress shortly before he died.

But at least one of his relationships was kept secret until after his death. Screenwriter William Bast met the actor when they were 19 and studying at UCLA. They became roommates and close friends, though the relationship went deeper than that, with William writing in his memoir Surviving James Dean that they kept the romantic facet of their relationship secret due to James' increasing fame.