The Rosemary's Baby actress recently admitted the 25-year-old journalist could "possibly" be the son of the legendary singer, who she was married to from 1966 to 1968.
She and Sinatra allegedly remained close after their marriage split - and gossips have long speculated that Ronan could be their lovechild. And now Farrow has fuelled the rumours as part of a new Vanity Fair interview, without going into details.
On Wednesday (02Oct13), Ronan addressed the rumours on Twitter, writing, "Listen, we're all *possibly* Frank Sinatra's son."
Ronan, a human rights activist, reportedly disowned Allen after he split from Farrow and began a relationship with her adopted daughter Soon-Yi Previn.
Sinatra died following a heart attack in 1998.