NEW YORK/LOS ANGELES (Reuters) - Britney Spears and former Disney Channel star Demi Lovato officially joined the U.S. version of The X Factor on Monday, leading creator Simon Cowell to pledge the second season would "seriously kick butt" when it returns in September.

Spears, 30, the world's biggest pop phenomenon of the 2000s, and Lovato, 19, will fill the judge's slots left open by Paula Abdul and Nicole Scherzinger, who were both fired by Cowell after a disappointing first season in 2011.