Dexter star Michael C. Hall has a secret new girlfriend and RadarOnline.com has exclusively learned she is 15 years his junior.

The 40-year-old actor has been having secret trysts with Vanessa Abrue, a Showtime staffer, and the two have been going to great lengths to keep the relationship low-key since they started dating in March - just three months after his ex, Jennifer Carpenter, filed for divorce from him.

"No one would ever suspect these two were together because she isn't what anyone would expect Michael to be with," the insider said. "They hardly go out together but when they do it's always with a group of people so no one would suspect he's with someone."