NEW YORK —  "Late Show" host David Letterman (search) has a secret joke writer — none other than the retired king of all late-night television, Johnny Carson (search).

The 79-year-old former host of NBC's "The Tonight Show" occasionally sends Letterman new jokes he has written and that Letterman sometimes uses in his nightly "Late Show" monologue, according to CBS senior vice president Peter Lassally, a onetime producer for both men.

The news that Carson was still active — and still favoring Letterman with his best stuff — came as a shock to those who believed that Johnny had turned his back completely on show biz.

Lassally says that while Carson has remained out of the public eye since retiring, he keeps up with late-night TV.

Lassally said Carson also keeps up with political news and other current events that were once fodder for his own "Tonight Show" monologue.

"I think the thing he misses the most is the monologue," Lassally said.

"He reads the newspaper every day and might think up five good jokes that he wishes he had an outlet for.

"Once in a while he sends jokes to Letterman and Letterman will use his jokes in the monologue and he gets a big kick out of that."

Letterman was out of town yesterday and could not be reached. Show officials yesterday said only Letterman knew which Carson jokes had made it on to the air.

Carson retired in 1992 after nearly 30 years as host of "The Tonight Show" and was replaced by Jay Leno (search). But Carson has always felt privately that Letterman, not Leno, was his rightful successor, Lassally said.

Letterman, who long hosted NBC's "Late Night" show immediately following Carson's program, jumped to CBS in 1993.