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Degas' 1877 painting Les Choristes, or The Chorus Singers, is done in pastels and depicts a line of men singing in the opera Don Juan.
The police were making random checks on a bus at a highway rest area about 30 kilometers east of Paris on Feb. 16 when they found it and the Orsay museum confirmed its authenticity, the culture and budget ministers said in a statement.
The statement gave no information about why the painting was on the bus or who left it there.
The French artist is most famous for his paintings and sculptures of dancers and is often associated with the Impressionist movement.
The painting is thought to be worth upwards of NZ$1.3m.