“Naty remained a committed revolutionary,” said Jonathan Hansen, a historian at Harvard who interviewed Ms. Revuelta several times for a biography that he is writing about the young Fidel Castro. Last May Ms. Revuelta, then 88, shared with him about 40 of the letters Mr. Castro had written her from prison.

“She told me she didn’t agree with everything that had been done, or the way it turned out, but she said, ‘Cuba had to have this revolution,’” Mr. Hansen said.

According to several Cuba news blogs, including Cafe Fuerte, which cited sources close to the family, Ms. Revuelta was taken to a Havana hospital last week and died there on Saturday. She had been in declining health since being seriously injured in a fall last summer.

Ms. Fernandez was with her when she died, Cafe Fuerte reported, and directed that her ashes be scattered at sea.

The romance between Ms. Revuelta and Mr. Castro was whispered about all over Cuba, and in 1998 it was detailed in the book “Havana Dreams,” by Wendy Gimbel. In a review in The New York Times, Alma Guillermoprieto, who had written extensively about Latin America, highlighted the fairy tale nature of the story of the stunning socialite who gave up everything — “the tea parties, the charity fashion shows in which she was always the star, the happy, crooning nannies and the starched and frilly babies — for some sweaty, bearded rabble-rouser who was, to boot, the illegitimate son of white trash!”

Ms. Revuelta was born into Havana society on Dec. 6, 1925. Her parents divorced soon after she was born, and she was reared by her mother, Natica. Ms. Revuelta was educated in private schools in Cuba and the United States. She was 22 when she married Dr. Fernandez, who was nearly twice her age. They had a daughter, Nina, and lived in an exclusive Havana neighborhood.

With her husband away much of the time, Ms. Revuelta found that life boring and yearned for adventure. She joined the Orthodox Party, which championed social justice causes. After the November 1952 coup by Fulgencio Batista, she, like many Cubans, became active in the opposition movement.