PARIS — You’ll find them up in the balcony, or in standing room, silently mouthing the libretto or humming along with the score. These are the superfans: the compulsive lovers of opera or ballet or theater who see every performance; who travel from city to city for Marilyn Horne or Mikhail Baryshnikov; who know every downbeat of “Così Fan Tutte” or “A Chorus Line.”

Most are harmless admirers. Some become lay experts. But the superfan can be conniving, as in “All About Eve,” or even murderous: the Tejano sensation Selena was killed by her fan club president. If great art stimulates the heart and the head, the superfan has the ratio out of whack: Passion wins out over reason, and appreciation tips into obsession.

In the annals of French art history, the superfan par excellence is Edgar Degas: the most Parisian of all the Impressionists, and an obsessive of the first magnitude over the opera and ballet. For decades, he watched the leading singers and dancers under the new electric lights, and scrutinized the young members of the corps de ballet in the wings and backstage. Close to half of Degas’s painterly output depicts the Opéra de Paris — which was (and still is) both an opera and a dance company, and which he knew as intimately as Monet knew Giverny’s gardens.

In the year 1885 alone, Degas went 55 times to the still-new Palais Garnier. He saw one opera, the now-forgotten “Sigurd” by Ernest Reyer, at least 37 times. His images of dancers making their grandes arabesques or bending at the barre, now schmaltzy stalwarts of dorm-room posters, were the projects of true mania.