For nearly six decades a Cézanne watercolor depicting Paulin Paulet, a gardener on the artist’s family estate near Aix-en-Provence, was familiar to scholars only as a black-and-white photograph. No one knew if the actual work, a study for Cézanne’s celebrated “Card Players” paintings, still existed and, if it did, who owned it.

But the watercolor recently surfaced in the home of a Dallas collector and is now heading to auction at Christie’s in New York on May 1, officials at the company said on Monday. It is estimated to sell for $15 million to $20 million.

Cézanne’s images of workers on his family farm — pipe-smoking men sitting around a table, their expressions dour, their dress drab, absorbed in a game of cards — are among his most recognizable works. Some are pictured alone; others are shown in groups of two or more. Paulet is the only one of the figures to appear in all five paintings in the “Card Players” series....