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The archive of letters from Cohen to Marianne Ihlen chronicles their 1960s love affair and the blossoming of Cohen's career from struggling poet to famous musician.
The top letter, in which Cohen wrote in December 1960 about being "alone with the vast dictionaries of language," fetched $US56,250 compared to an original high estimate of $US10,000.
A 1964 letter, in which Cohen wrote "I am famous but empty," went for $US35,000, Christie's auction house said on Thursday.
Cohen and the Norwegian-born Ihlen met on the Greek island of Hydra in 1960 and she became the inspiration for several of his best-known songs, including Bird on a Wire, Hey, That's No Way to Say Goodbye, and the 1967 track So Long, Marianne.
Ihlen died of leukemia in Oslo in July 2016, aged 81. Cohen, who was also suffering from leukemia, died in November 2016 at the age of 82.
The letters were sold by Ihlen's family. The buyers were not revealed.
The top lot in the five-day online auction was an Italian bronze bell dating from the 15th or 16th century that hung in the Hydra home that Cohen and Ihlen once shared.
It realised $US81,250 compared to a pre-sale estimate of up to $US12,000.
The bell is believed to have inspired the lines, "There is a crack, a crack in everything" in Cohen's 1992 release Anthem.