The charcoal sketch on watercolor paper shows Lady Gaga reclining nude, signed by "Benedetto," the name Bennett signs all his art works with.
The 85-year-old jazz singer, who earlier this year recorded "Lady is a Tramp" in a duet with Lady Gaga for his album "Duets II," posted on Twitter that he was "thrilled" his portrait was up for auction.
Bennett, 85, sketched the picture while Lady Gaga, 25, posed for photographer Annie Leibovitz for the cover of Vanity Fair magazine's January issue.
Currently, the highest bid is $6,700, and is expected to rise during the course of the auction, which will end on Dec. 19, with all proceeds going to Bennett's Exploring the Arts charity and Lady Gaga's Born This Way Foundation.
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