Paris-based auction house Tajan announced the discovery of "The Martyred Saint Sebastian” drawing Monday. This is the first work by da Vinci to be discovered in over 15 years, according to Tajan, which has valued the drawing at $15.8 million, Fox News reported.

The sketch is one of 14 unframed drawings brought into the auction house in March by a retired doctor, the New York Times reports. The drawings had been collected by the man’s father.

Thaddee Prate, Tajan’s director of old master pictures, realized that a drawing that depicts St. Sebastian tied to a tree was noteworthy. Prate sought a second opinion from independent art dealer Patrick de Bayser, who noticed that the drawing was by a left-handed artist (da Vinci was left handed). De Bayser also found two small scientific drawings of candlelight on the back of the sheet, accompanied by minute notes, according to the New York Times.

Leonardo da Vinci's scientific drawings on the back of the sheet depicting St. Sebastian (Tajan).

Tajan then reached out to renowned da Vinci authority Carmen C. Bambach, curator in the Department of Drawings and Prints at the Metropolitan Museum of Art in New York, who confirmed the drawing’s authenticity.

"This is an exciting new discovery of an authentic double-sided sheet by the master (1452-1519), representing on the recto the full figure of the martyred Saint Sebastian tied to a tree in a landscape, and on the verso, notes and diagrams about light and shadow, which relate to Leonardo’s study of optics,” said The Metropolitan Museum of Art, in a statement.