Letterman’s wife, Regina Lasko, had no clue the comic was having an affair with his pretty, much younger assistant Stephanie Birkitt — a $200,000-a-year employee — when she was tagging along on the cozy, romantic trips, sources said.

Lasko, 48, “has to be really, really pissed” to learn recently that Letterman was so brazen as to have invited his paramour on vacations with their 6-year-old son, Harry, a source said.