She signed off as "Country Mouse" and called him "My most excellent boss."
She was Hollywood star Katharine Hepburn and he was eccentric entrepreneur Howard Hughes. Many of her letters to him are going up for auction in Los Angeles for the first time on July 23.
The emerald and diamond engagement ring Hughes gave to Hepburn in 1938 is also for sale, and is expected to fetch between US$20,000 – US$30,000 (S$27,850 – S$41,770) at the Profiles in History auction.
Brian Chanes, head of consignment relations at Profiles in History, said the items were rare because of their personal nature.
"Very little has really eked out because he (Hughes) was such a private individual," Chanes said. "We have over 55 hand-written letters, love notes, bedside notes from Katharine Hepburn. They were lovers for about a year, a year and a half, from 1937 to '38 and they’re very, very juicy."
Hughes and Hepburn never married but their romance, which was chronicled in the 2004 movie The Aviator, was a huge celebrity story at the time.
The letters are expected to fetch up to US$15,000 at the auction.
Hepburn used pet names and pseudonyms like C. Mouse and Mrs HR Country when signing the letters, which showered the movie mogul and millionaire businessman with praise.
"My most excellent Boss" begins one. In another, "The Philadelphia Story" actress called him "brilliant as Einstein," and "graceful as a gazelle."
The letters and engagement ring are part of a wider collection of Hughes' documents and personal belongings that were acquired from the family of the late Vernon C Olson, who was private accountant to Hughes.
Other items include the two-tone jacket Hughes wore while piloting his huge Spruce Goose flying boat, his trademark fedora hat and contracts for multiple movies.
(Reporting by Rollo Ross and Jill Serjeant; Editing by Dan Grebler)