Katy Perry and Avril Lavigne to guest judge American Idol

Music stars Katy Perry and Avril Lavigne will be guest judges on American Idol during the final two days of auditions held this week in Los Angeles. With each star judging on a different day of L.A. auditions, Katy and Avril will join Simon Cowell, Randy Jackson, and Kara DioGuardi at the judges' table.

Katy and Avril aren't the first stars to guest judge at Season 9 auditions. Recently, Victoria Beckham served as a guest judge in Denver and Boston, Mary J. Blige shared her expertise in Atlanta, Joe Jonas and Neil Patrick Harris joined the panel in Dallas, Kristin Chenoweth met up with the show in Orlando, and Shania Twain was the guest judge in Chicago.

The Grammy-nominated Katy Perry has captured the hearts of critics and audiences alike. Awarded Best New Act at the MTV Europe Music Awards and voted Best New Artist by readers of Rolling Stone, Katy is currently wrapping up her sold-out "Hello Katy" worldwide tour in September. Katy even performed during a Season 8 results show. See photos of Katy's American Idol performance.

Avril Lavigne has been in the industry since she was 17 years old. Her first album, Let Go, sold more than 16 million copies worldwide and earned her eight Grammy Award nominations and four Juno Awards. With her three albums accounting for over 30 million records sold worldwide, Avril is currently in the studio working on her fourth album. In addition, she designs her own line of clothing, Abby Dawn, and she just launched her first fragrance, Black Star.

American Idol will kick off its ninth season with the audition episodes this January on FOX.

'True Blood' star Evan Rachel Wood reveals huge crush on Adam Lambert

Katy Perry isn't the only female celebrity who has a crush on Adam Lambert.

Evan Rachel Wood told Fox News that she has the hots for Lambert in a major way. The 21-year-old actress has a penchant for androgynous men and has been romantically linked to singer Marylin Manson.

Wood said,
"I probably couldn't help myself, that guyliner is so amazing. But he's got an incredible voice, talent is very sexy. I love that he's so androgynous, he just doesn't care, has fun and wears whatever he wants."