Actress Holmes reportedly met McPhee at Los Angeles' Church of Scientology, where the singer allegedly attended courses with her boyfriend, and has since become a huge fan of the pretty brunette.
And, after McPhee lost out to Taylor Hicks in the Idol finale last week, the celebrity couple got in touch with the California girl and asked her to perform at their wedding ceremony.
A source close to the couple says, "Tom and Katie have become Katharine's biggest fans. They think she's destined to become a singing legend."
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