When Belgian art dealer Klaas Muller snapped up a potential sleeper Old Master at an online auction three years ago, he got more than he bargained for. It appears the work isn’t just a rediscovered Peter Paul Rubens (1577–1640), as he had secretly hoped—it’s actually two compositions in one, with a hidden second image appearing if you flip the work upside down.

At first glance, the painting appears straightforward: an elderly man with a long, bushy beard, his bald head illuminated against a dark background. But turn the painting 180 degrees, and a second face emerges, of a young woman peeking through the flowing facial hair.

A study of an old man believed to be by Peter Paul Rubens, with a side-by-side comparison showing it upside down, where a woman’s face can be seen hidden in his beard. Photo: Klaas Muller/Brafa Art Fair.

“I wasn’t sure it was a Rubens; I just knew it was very Rubens-esque, so it was still a gamble,” Muller told the Guardian of the decision to buy the work from a “lesser-known auction house in northern Europe.”

The auction house had identified it only as a study on paper by an unknown artist of the “Flemish school.” But something about it inspired Muller to do some more research.

“It immediately caught my attention,” Muller told Der Standaard, which first announced the discovery. “It was a real character piece. He could have been the apostle Peter or Neptune. I could see that the work was of high quality. The character seemed familiar to me.”

Soon, Muller realized why he had recognized the painting of the old man. The same figure of the bearded old man with the downturned head could be seen in Rubens’s series, “The Twelve Apostles,” commissioned by the Duke of Lerma and now in the collection of Madrid’s El Museo del Prado. Muller’s auction sleeper was a dead ringer for Rubens’s St. Thomas, in which the saint glances down to read from a heavy book, presumably the Bible.

Peter Paul Rubens, Saint Thomas from “The Apostles.” Collection of El Museo del Prado, Madrid.

And the same model seemed to pop in other Rubens compositions. He looked just like Saint Amandus in the altarpiece The Raising of the Cross at Antwerp Cathedral, the Pharisee behind Jesus in The Tribute Money at San Francisco’s Legion of Honor, and King Melchior in The Adoration of the Magi, also in the Prado.

Excited about the work’s potential, Muller registered for the auction and placed the winning bid, for what he described as a “reasonable price” under €100,000 ($115,000).

Peter Paul Rubens, The Adoration of the Magi. Collection of El Museo del Prado, Madrid.

The work will make its debut later this month at BRAFA, the Brussels art and antiques fair, where Muller is the chairman. He hopes he can interest a museum in hosting the work in a long-term loan.

The oil-on-panel painting dates to between 1610 and 1612, which would make the work on paper a study for it, circa 1609.

The fact that the subject reappears in different Rubens paintings suggests the work may have been something of a prototype for the artist, as he worked to create a set of different physiognomies to plug into his scenes.

To confirm his hunch, Muller called up art historian Ben van Beneden, a former director of the Rubenshuis, or Rubens House, in Antwerp.

“I think it’s very likely,” Van Beneden told the Guardian. “You have to be cautious because you are dealing with a painting that wasn’t made for the market but as a working material. But the craftsmanship is outstanding—it has a very lifelike quality.”

Peter Paul Rubens, The Tribute Money (detail) (c. 1614). Photo: Francis G. Mayer / Corbis / VCG via Getty Images.

Other art historians, however, are calling for caution.

“I won’t comment on the authenticity of the painting in question,” Nico Van Hout, head of collections at the Royal Museum of Fine Arts Antwerp (KMSKA), and the author of a book on Rubens’s study heads, told the Gazet van Antwerpen. “I only do that for works I have seen myself.”

“We constantly receive requests, not only from our own country, but from all over the world, asking us to give our opinion [on unauthenticated paintings],” he added. “These are all people who think or hope that they have won the lottery. But the chance that they have a real masterpiece in their possession is microscopically small.”

Van Hout’s book did note that there was a Rubens study for this particular old man listed in the artist’s studio catalogue. That means the missing study could be the work on paper Muller bought at auction.

But while it is still too early to definitively confirm the painting as an autograph Rubens, it is possible to draw some conclusions about the work. The unique piece probably wasn’t meant as an optical illusion. The hidden woman was probably painted first, and then the artist decided to reuse the paper, hiding her visage in the new composition.

BRAFA will be on view at Pl. de Belgique 1, 1000 Brussels, Belgium, January 25–February 1, 2026.