In early 1970, at the age of 23, Duke had an affair with 17-year old Desi Arnaz, Jr.. The couple's relationship became tabloid fodder, due in part to the vocal and public opposition of Arnaz's mother, Lucille Ball. By late spring, Duke and Arnaz had broken off their relationship, and she began dating actor John Astin, who was 16 years her senior. Around the same time, Duke developed an intimate relationship with rock promoter Michael Tell. In June 1970, in the midst of a manic phase, Duke learned she was pregnant. Unsure whether Arnaz or Astin was the father of her unborn child, Duke married Michael Tell on June 26, 1970 in order to "give (her child) a name". Their marriage lasted 13 days before ending in an annulment on July 9, 1970; Duke claimed the marriage was never consummated. Her son, actor Sean Astin, was born on February 25, 1971. Though Duke said in her 1987 autobiography that John Astin was Sean's biological father, she later stated that she had always believed that Desi Arnaz, Jr. was Sean's actual biological father. It turned out that neither statement was correct; in 1994, Sean Astin underwent biological testing to determine his paternity, and the results showed that Astin's father is actually Michael Tell.