"Though 25 years apart in age, they had an affair for about 15 years. Ms. Branden knew about it because Ms. Rand had insisted from the start that both spouses — Ms. Branden and Ms. Rand’s husband, Frank O’Connor — give their consent. Ms. Branden revealed the story, she said, partly to set the record straight about Ms. Rand’s sudden banishment of the Brandens from her circle in 1968. She had denounced them in a widely circulated essay, claiming they had exploited her financially. The couple denied the accusation."
From the obituary for Barbara Brandon, the author of "The Passion of Ayn Rand."
ADDED: Here's a clip of Brandon talking about her book on Reason TV:
I found that looking for a good clip of Helen Mirren playing Ayn Rand in the Showtime movie based on Brandon's book. Here's the trailer for that (which I watched and which caused me to say "Oh, bullshit.")