During an interview on Howard Stern's Sirius Satellite radio show, Arquette said he didn't know if his wife and her Cougar Town co-star Brian Van Holt were getting physical, but acknowledged an 'emotional affair', reports the New York Daily News.
He added, "Part of the terms of our separation was that I was allowed to see other people and so was she."
RadarOnline.com previously reported that the nature of the pair's relationship was questioned in April after Cox sent her chauffeured limo home and opted to spend the night on the Cougar Town set with Van Holt.
"The talk on the set was that she and David had an open marriage. David came to the Cougar Town set maybe four or five times over the course of the season, that's not a lot compared with other actors," said an on-set source
However, the co-star's relationship may be nothing but platonic, said Van Holt's sister, Heather.
"As far as I know they are not romantically involved.
"We are a very close-knit family and he is usually very open with me, I'm sure he would have told me," she said.
Arquette and Cox had been together for 11 years.