David Letterman continued his affair with CBS staffer Stephanie Birkitt much longer than he has admitted - even carrying on after his marriage last March, alleged extortionist Robert Halderman charged in court papers yesterday. Halderman confronted Birkitt in December 2008 with evidence of the affair and she promised to end it, the court documents say. But Halderman’s lawyer, Gerald Shargel, claims the relationship continued “unabated” into this past summer. Letterman’s rep has said the affair ended before Letterman’s marriage in March. Halderman made the claims in a motion to dismiss the extortion case. He claims authorities were swept up in “the sensationalism surrounding the indictment.” Letterman says Haldermain demanded $2 million to keep quiet about the affair. Halderman’s lawyer said the demand was “a pure commercial transaction” over a screenplay.