What's more, the auction house estimates "The Martyred Saint Sebastian" could be worth as much as 15 million euros ($16 million).
"The doctor couldn't have imagined there could have been something so important in his portfolio," says Thaddée Prate, director of Old Masters paintings at the French boutique auction house.
"Discoveries happen, yes, but with lower importance, of course. Leonardo, I think this will be only once in a lifetime."
"The Martyred Saint Sebastian" depicts the early Christian saint bound to a tree. This new discovery is the third known da Vinci sketch of the subject. The artist mentioned eight drawings of the saint in his Codex Atlanticus.
The doctor, whose identity has not been disclosed, brought 14 drawings inherited from his father to Prate's Paris office in March. He'd had the drawings for years, but only after retiring did he resolve to have them valued.
Prate assumes the doctor's father, a book collector, acquired the drawings at a bookstore in the early 20th century, as they had all been mounted on paper mats rather than framed.
Intrigued by the drawing's apparent age, Prate enlisted Patrick de Bayser
, an expert in the valuation of Old Masters drawings, to confirm the artist. The paper frame had the name Michelangelo, but the expert doubted that was the artist of this particular drawing.
Noticing the traces of markings on the back, de Bayser removed the drawing from its backing, revealing diagrams of light and shadow and notes written right-to-left -- further suggesting a left-handed artist like da Vinci.
Soon after, Carmen C. Bambach, curator of Italian and Spanish drawings at the Metropolitan Museum of Art in New York and the world's pre-eminent da Vinci authority, confirmed the attribution, describing it as "quite incontestable" and an "open-and-shut case" in an interview with the New York Times
. She estimates the drawing was completed between 1482 and 1485.
A new record?
If all goes according to plan, Rodica Seward, president of Tajan, says "The Martyred Saint Sebastian" will hit the auction block in early June as part of its annual Old Masters sale, with an estimated price of 15 million euros ($16 million) -- $4 million more than the current record for a da Vinci sketch, set in 2001 at a Christie's auction.
"It's not comparable (to the other drawings)," Seward says. "This is a very unusual drawing, it's very complete. It's got a background and scenery behind, which none of the other Saint Sebastian drawings have. This is an incredible discovery."
"(Da Vinci is) the most important artist from all over the world, I think, from any time. Each little point we can add to his work is very important for our knowledge of the artist and his work," Prate added.
But before that auction can go forward, there are a few uncertainties to be overcome. The drawing must first be approved for a passport, allowing it to be sold internationally.
Until then, French national museums have the opportunity to preempt the sale, negotiating with the house directly under the claim that it's a national treasure.
But Seward is hopeful that it will be opened to the international community.
"We applied for the passport," she says. "Now it's in God's hands."