Stewart and Pattinson have been dogged by reports that they are dating ever since they first appeared onscreen together in last year's vampire love story.
The romance rumors went into overdrive when they started shooting the sequel New Moon in Canada earlier this year, but the pair has always denied they're anything but close friends and co-stars.
Pattinson has also previously been linked to another co-star, Nikki Reed.
And Garth, whose husband Facinelli plays a father figure to Pattinson's character Edward Cullen in the films, admits the actor is indeed romancing one of his co-stars -- but refuses to reveal which one.
She tells the New York Daily News, "I can't say (who it is), because that would be breaking my promise (to husband Facinelli), but he (Pattinson) is dating one of them."
(c) 2009 WORLD ENTERTAINMENT NEWS NETWORK LTD. All global rights reserved. No unauthorized copying or re-distributing permitted.