Belgian art dealer Klaas Muller bought a rare study painting by Peter Paul Rubens for less than €100,000 at a northern European online auction. This painting cleverly shows a duck-rabbit optical illusion, featuring both an old bald man with a bushy beard and a young woman with braided hair. Muller only guessed it might be a Rubens at first, calling it a "gamble". After cleaning and expert study by art historian Ben van Beneden, confidence grew that it is a genuine Rubens. Van Beneden said, "The craftsmanship is outstanding – it has a very lifelike quality." The old man figure appears in several famous Rubens works, including The Raising of the Cross and The Adoration of the Magi. Muller suspects the painting is a prototype study never before seen and is excited to show it publicly. The artwork will be displayed at the Brafa art fair in Brussels from 25 January. Muller hopes a museum will take it on loan to share with a wider audience. "Even as a study, it deserves as wide an audience as possible," he said.