A charcoal drawing of a nude woman may have been sketched by Leonardo da Vinci as he prepared to paint the Mona Lisa.
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And they believe that it was done "at least in part" by Leonardo, pointing to almost identical similarities in the face, hands and body.
One expert says it was "almost certainly" a preparatory sketch for an oil painting, and it was likely worked on "in parallel" with the famed Mona Lisa 500 years ago.
The "Monna Vanna" was attributed to Leonardo when it was sold to the Conde Museum, north of Paris, in 1862.
Mathieu Deldicque, a curator at the Louvre, where the Mona Lisa is displayed, told AFP: "The drawing has a quality in the way the face and hands are rendered that is truly remarkable. It is not a pale copy.
"We are looking at something which was worked on in parallel with the Mona Lisa at the end of Leonardo's life.
"It is almost certainly a preparatory work for an oil painting."
He said the hands and body in the drawing are almost identical to the Mona Lisa, and the sketch is almost the same size.
Mr Deldicque also argued that it may have been traced onto a canvas, pointing to small holes around the figure of the nude woman.
Bruno Mottin, another expert from the Louvre, told the Parisien newspaper that the drawing of "very high quality" dates from Leonardo's lifetime, and was an original.
But he cast some doubt on hopes that it was done by Leonardo - who died in France in 1519 - saying the master was left-handed and the hatching near the head was done by a right-handed artist.