Questions over Ronan's parentage have been around for years, and now Mia has admitted that the iconic singer may "possibly" be the father.
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In a new interview with Vanity Fair, the 68-year-old said that Frank was "the great love of her life", saying, "We never really split up."
Asked pointblank if her son with Woody might actually be the son of Frank, she replied, "Possibly". No DNA tests have been done.
When Vanity Fair asked Frank's daughter Nancy Sinatra Jr about Ronan being treated like a member of her family, she replied, "He is a big part of us, and we are blessed to have him in our lives."
Frank Sinatra and Mia Farrow on their wedding day
Mia and Frank were married in 1966 in Las Vegas when she was 21 and he was 50. Just 18 months later, their marriage fell apart and divorce followed in 1968.
Two years later, she married German-Austrian pianist Andre Previn. The couple had three biological children — twins Matthew and Sascha and Fletcher — and adopted three more; Vietnamese infants Lark Song and Summer 'Daisy' Song, and Soon-Yi. They divorced in 1979.
In 1980, Mia started seeing famed film director Woody Allen. Together they adopted Misha and Dylan, and in December 1987, Mia gave birth to son Satchel, known as Ronan.
During their long-term romance, Mia became Woody's muse and starred in 13 of his films, including Hannah And Her Sisters.
Their relationship ended in 1992 when she discovered he was having an affair with her oldest adopted daughter, Soon-Yi Previn, then 22. They were married in 1997, and have since adopted two children.