Is Katie Cheating On The Piano Man?

Katie Lee Joel, the much younger wife of Billy, doesn't really look as if she's too broken up over the fact that her man is on tour.

Could it be because she's got a little sumthin sumthin on the side?

The National Enquirer reports that the 27-year-old may be be shtuping an Israeli fashion designer almost half Billy's age!

"Billy is furious," a friend told the tab. "His wife's relationship with this guy is causing a split in his marriage. Initially, Billy thought it was great that she met a new friend, someone to occupy her time while he trudges through a long tour schedule. But now he's become very concerned."

Looks like Billy's on tour until November...think Katie will stick around til then?