Promotions considered as Letterman trails Leno

? David Letterman’s new contract with CBS requires the network to throw extra promotional weight behind his late-night program, bowing to Letterman’s hope that will help narrow the gap between his show and ratings leader “The Tonight Show With Jay Leno.”

Yet after a tumultuous 10 days in which Letterman’s possible defection to ABC commanded national headlines, late-night ratings this week have been virtually unchanged, with Leno beating “Late Show With David Letterman” all three nights for which ratings are available  including Monday, when Letterman returned from vacation and announced on the air his intent to stay at CBS.

Analyzing data from 50 major cities monitored by ratings service Nielsen Media Research that account for more than half of U.S. households (national estimates for late night won’t be available until next week), Leno has averaged a 4.6 rating and 12 share this week, meaning 4.6 percent of all homes and 12 percent of TV sets in use during that hour were tuned to “The Tonight Show.”

Letterman enjoyed a minor spike Monday, with a 3.9 rating and 10 share, but dropped the next two nights and averaged a 3.6 rating Monday through Wednesday, mirroring his average this season.

Those results reinforce the sense that shifts in late-night viewing habits tend to be glacial, raising questions about how much effect additional promotion is likely to have  especially as long as NBC continues to beat CBS from 10:30 to 11 p.m., leading into late local newscasts.

For the season, Leno is averaging 6 million viewers per night nationally, compared with 4.3 million watching Letterman.

Letterman’s contract calls for CBS to leverage assets from parent company Viacom to promote the show. Viewers can also expect to see extensive “Late Show” promotion during CBS’ coverage of the NCAA basketball tournament.