Letterman to simulcast on radio:

Starting Nov. 11, the "Late Show with David Letterman" will be simulcast each weeknight on 15 major-market radio stations owned by the Infinity Broadcasting network, a corporate sibling of CBS, marking a new avenue of cross-promotion for Letterman. Both CBS and Infinity are owned by Viacom Inc. .

More stations are expected to follow suit in the near future, Infinity said.

The radio deal offers a new avenue of cross-promotion for Letterman as CBS battles to close the ratings gap between its Emmy Award-winning late-night host and his front-running NBC rival, "The Tonight Show with Jay Leno."

"We feel this is a great opportunity for David Letterman's millions of fans to listen to the show while they're driving in their cars or away from their homes," Infinity's chairman and CEO, John Sykes, said in a statement.

CBS vowed to beef up "Late Show" promotion on various Viacom properties, including MTV and Infinity, when it renewed Letterman's contract this spring, fending off a highly publicized bid by ABC to lure away the gap-toothed comedian.

CBS also has boosted the size of the audience tuning in for Letterman on Mondays and Thursdays, thanks to two new 10 p.m. dramas — "CSI: Miami" and "Without a Trace" — that provide stronger "lead-ins" to local news preceding the "Late Show."

So far, however, Leno still holds a decisive lead, averaging 5.9 million viewers so far this season compared with 4.1 million for Letterman.

While it is rare for network television shows to be simulcast over the radio nowadays, it is not unheard of. The venerable CBS news magazine "60 Minutes" currently airs on 18 Infinity radio stations.