Credit: Mark Liddell

Avril Lavigne and Katy Perry will join the growing list of singers and entertainers serving as guest judges on season 9 of American Idol, has learned exclusively. Sources close to the show say Lavigne and Perry will each spend one day critiquing singing hopefuls when the show films its callback audition episode in Los Angeles; the latter singer was confirmed as an Idol sub several weeks ago, but had not been scheduled. Other names confirmed to fill the seat formerly held by Paula Abdul during season 9 include Victoria Beckham, Mary J. Blige, Neil Patrick Harris, Shania Twain, Kristin Chenoweth, and Joe Jonas.