The late industrialist Armand Hammer engaged in a longtime extramarital affair with the woman who is now chief fund-raiser at the Westwood museum that bears his name, according to newly filed documents in a bitter lawsuit over legal title to Hammer's art collection.

In 1974, the court documents state, Hammer even had the woman named curator of art of Occidental Petroleum Corp., which Hammer headed from 1957 until his death at the age of 92 last Dec. 10, even though the oil company at the time allegedly had no art holdings.

The allegations of longtime infidelity by the legendary Hammer are contained in court filings made on behalf of Joan Weiss, the niece of Hammer's wife, Frances. Frances Hammer died in December, 1989. The Hammers were married in 1956.

In the litigation, Weiss seeks to gain half ownership of Armand Hammer's art collection. She contends that Frances Hammer was improperly induced by Armand Hammer, Occidental and various lawyers to sign away her interest in the paintings, drawings and sculptures.

The allegations of a sexual relationship involve Hammer and Hilary Gibson, now chief financial officer at the Armand Hammer Museum of Art and Cultural Center. When the alleged affair began in the early 1970s, Gibson was known by another name, Martha Wade Kaufman.

The Weiss documents allege that the legal name change "reflected the desire of both Martha Kaufman and Armand Hammer . . . to conceal from Frances Hammer (the fact) that he had selected Martha Kaufman to hold a high-ranking position at the Armand Hammer museum."

Amy West, a spokeswoman for the Hammer museum, said Gibson would have no comment on the charges. Frank Ashley, an Occidental Petroleum spokesman, said the company had no comment except that Gibson is not currently on the Occidental payroll.

Stephen Garrett, the museum director, said in an interview Friday that the museum "had no reaction except sorrow" over the emergence of the sexual allegations. "I'm very sad that (the alleged details of the affair) should be there" in the court document, Garrett said. "Sad for the museum, primarily, but for Hilary too."

The court filing--made late Wednesday by attorneys for Weiss--alleges that the relationship between Armand Hammer and Gibson haunted Frances Hammer in the final years of her life. Other legal documents obtained by The Times and interviews with people familiar with the situation indicate that Frances Hammer even considered separating from her husband.