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The Leonardo has been valued at €15m and the Fragonards at €6m but Tajan cannot yet reap the commercial benefit of their discoveries. Their status as national treasures means that a 30-month period must be allowed to elapse to give a national museum the option to acquire them.

The Leonardo, which depicts the martyrdom of Saint Sebastian, with two scientific studies on the reverse, was bought into the auction house last year by an unnamed client as part of a group of 15 unframed drawings.

Subsequently attributed to Leonardo in consultation with Dr Carmen C Bambach, curator in the drawings and prints department at the Metropolitan Museum in New York, it is the first work on paper to be ascribed to the Italian master since 2000.

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The Leonardo drawing which depicts the martyrdom of Saint Sebastian which was consigned to Tajan.

The Fragonards came to light when Thaddée Prate, Tajan's Old Masters director, was doing an inventory in a Normandy Chateau. The auction house has been additionally assisted by the Paris experts cabinet Turquin.

Titled Le Jeu de la Palette and Le Jeu de la Bascule, both compositions, which measure 2ft 5in x 3ft ( 75 x 93cm), depict young people playing outdoor games in landscape settings. Both works once belonged to Pierre Jacques Onésyme Bergeret de Grancourt.  He was an amateur artist and contemporary of Fragonard who accompanied him on his second trip to Italy from 1773-74 and assembled an impressive collection of art.

Tajan's two works are thought to have been painted in France after 1761. The last known reference to the paintings is in the auction in Paris of Bergeret de Grancourt’ s collection in 1786, the year after he died.