Around 5.8 million viewers tuned into his late night program Thursday when he announced he had engaged in sexual affairs with his female employees. His show Monday - during which he apologized to wife Regina Lasko - attracted 5.7 million viewers.
Birkitt, 34, and Halderman, 51, lived together from early 2005 until August of this year at his Connecticut home.
In December 2008, Halderman found a diary, in which Birkitt detailed her fling with Letterman, according to the New York Post. When Halderman confronted her, she insisted they were just friends. Halderman got angry, the Post reported, because he didn't believe her.