Heche gave birth just a few days after she finalized her divorce from Coley Laffoon. Laffoon and Heche married in 2001 and had a son, Homer, 7. Laffoon was a cameraman and began dating Heche shortly after her breakup with talk show host Ellen DeGeneres.
"Homer is very excited to be a big brother," the source tells Us. A rep for the couple says they are thrilled about the birth of Atlas.
Heche and Tupper first met in 2006 on the set of their short-lived ABC drama "Men In Trees." Heche was still married to Laffoon at the time.