The director of the acclaimed Holocaust documentary 'Shoah' said he has been forced to part with the correspondence because of a "scandalous" French inheritance law which means that they must go to her family on his death.
The letters, which are filled with the "mad passion" the couple shared during their seven-year affair in the 1950s, have never been published.
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They were bought by Yale University, which already holds de Beauvoir's manuscripts and personal archives.
"I never planned for these letters to come out or be published," said 93-year-old Lanzmann, who was the secretary of de Beauvoir's long-term lover, the philosopher and playwright Jean-Paul Sartre.
The golden couple of French mid-20th century intellectual life had a famously open relationship, and enjoyed - and endured - a number of similar love triangles.
Lanzmann, who was 18 years de Beauvoir's junior, fell in love with her while he was editing "Les Temps Modernes", the ground-breaking review she and Sartre founded after World War II, which the film-maker still heads.
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Agnes Poirier, author of "Left Bank", a new book about how "the ideas that shaped the modern world" were formed in the French capital during the intellectual tumult of the 1940s, said Lanzmann was the only man that de Beauvoir lived with.
"She and Sartre always kept separate apartments, but she let Lanzmann move in with her. He was about 26 she was 44 when the affair started, and he always said was she a 'grande amoureuse', a very passionate lover," she said.
"After the age of 40 de Beauvoir thought she was not desirable anymore but she had a second youth with him," Poirier said - and the author of the "The Second Sex" lived it "like a rebirth".
Poirier said that it had been always rumoured that Lanzmann "seduced her for a bet, or at least boasted that he could steal a kiss," but said that there was no doubting the intensity of their love.
"They had two little desks and they would work together in the mornings, then in the afternoons she would go and write with Sartre."
Just as with Sartre, it was an open relationship "but de Beauvoir took it badly when she discovered that Lanzmann had had an affair he didn't tell her about."
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"She wasn't judgemental, it was just the fact that he didn't tell her that annoyed her," the writer added.
According to Yale's library, which for now is only making the letters available in its reading room, most were written while de Beauvoir was travelling with Sartre on their headline-making visits to Russia, China, Japan and Cuba.
Lanzmann railed against the French law which he said had forced him to sell the letters to Yale, saying it was crazy that it "states that the contents of the letters did not belong to the person they were addressed to."
However, he said he had the right "to pass them on in the hope that the purchaser can, if not publish them, then at least conserve them and make them available to historians and researchers."
The top American university can "now be proud of having all of her letters to me", which Lanzmann called "an exceptional, passionate correspondence".
Christie's auction house which arranged the private sale did not reveal how much the letters had been sold for.
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