Katie Couric will celebrate her 15th anniversary as co-host of NBC’s “Today” show Wednesday and also may announce that she will be leaving the show on which she became a $15 million-a-year star.

Those who have been avidly reading the last year’s worth of speculation about CBS’s courtship of Ms. Couric to be “CBS Evening News” anchor will want to tune in to “Today” on Wednesday and to be especially alert during the second half-hour, which is thought to be when the most emotional moments will play out.

Meanwhile, by Tuesday the odds on possible successors to Ms. Couric at “Today” increasingly were stacked in favor of Meredith Vieira, the former “60 Minutes” correspondent who is a co-host of ABC’s “The View” and host of “Who Wants to Be a Millionaire.”

Weekend “Today” co-host Campbell Brown, while highly regarded by NBC News, is said to be out of the running.