Madonna and Zac Efron have now joined the list of celebrities who have reportedly been approached to be judges on “The X Factor” U.S. The show’s executive producer/judge Simon Cowell recently named several celebrities (including Mariah Carey, Paula Abdul, Nicole Scherzinger and George Michael) as being among those who have been in discussions to be a judge on “The X Factor” U.S.

According to the Daily Mirror: “Cowell, 51, is now pinning his hopes on Zac Efron and is also talking to Madonna and Paula Abdul.”

The Daily Mirror is also reporting that Cee Lo Green was in discussions to be a judge on “The X Factor” U.S., but instead he chose to be a coach on NBC’s rival singing contest “The Voice,” which premieres April 26, 2011. “The X Factor” U.S. premieres sometime in September 2011 on Fox. Auditions begin March 27, 2011, at the Los Angeles Sports Arena. Each winner of “The X Factor” U.S. gets a grand prize of $5 million and a record deal with Sony Music.

Cowell is the only “X Factor” U.S. judge officially announced so far. There are three other people who are supposed to be on the show’s judging panel. The “X Factor” judges act as mentors to contestants in the following four categories, with each judge getting a selected category: young female solo singers, young male solo singers, vocal groups and older solo singers.

Update: Zac Efron, who rose to fame because of the “High School Musical” franchise, has said in interviews that he now wants to focus on non-musical projects, so he may not be interested in being an “X Factor” judge. Efron’s publicist has already denied rumors from December 2010 that Efron would be hosting “The X Factor” U.S., so it is unlikely that Efron would want the more time-consuming job of being a judge on the show. [Source]