This is getting interesting...
And the No. 1 reason David Letterman is a cad: The "Late Show" host brought the assistant he was sleeping with on Caribbean vacations with his wife and their young son, sources told The Post.
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.
At least one jaunt was to the Caribbean island of St. Barts.
Lasko -- a former show staffer herself who married Letterman in March after 23 years as his girlfriend -- believed that Birkitt was only an innocent "friend" and "co-worker" at the time. Letterman also brought other friends and co-workers along for the ride.