The singer pens a "special song" for Aniston's 40th birthday

By KC Baker
Updated February 10, 2009 10:10 AM
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Lester Cohen/WireImage; National Photo Group

“Happy Birthday” isn’t the only tune Jennifer Aniston will be getting for her Feb. 11 birthday.

John Mayer has prepared his own serenade to help his girlfriend celebrate the big 4-0.

“He did write a special song just for her for her birthday,” a source tells PEOPLE of the Grammy winner. No word yet on his performance!

Think you know Jen well enough to write your own birthday tune? Take PEOPLE.com’s Jennifer Aniston birthday quiz!