A pair of small portraits made, perhaps spontaneously, of Rembrandt van Rijn's distant in-laws were discovered recently among a private collection.

Never seen before, or known in any of the Rembrandt literature, the portraits could be the smallest he ever made, and greatly surprised the owners who had never imagined the true origin of the inherited pieces.

If previous sales are any indication, they could bring in between $6.25 million $10 million (£5 million and £8 million) at auction.

Dated to 1635, the paintings depict wealthy plumber Jan Willemsz van der Pluym and his wife Jaapgen Carels, who lived in the city of Leiden. Their son, Dominicus van der Pluym, married Rembrandt's cousin, but rather than this being a distant and unnurtured inter-family connection, experts at the Rijksmuseum in Amsterdam say the nature of the portraits show there probably was great affection between the families and the painter.

The family of van der Pluym held onto the portraits until about 1760, until they were sold by a descendent to Polish buyers, who then sold them to French buyers, who then sold them to the 1st Baron of Glenlyon, James Murrary.

"The pictures were immediately of terrific interest," Henry Pettifer, international deputy chair of Old Master paintings at Christie's auction house, told CNN, adding that the owners were also taken by surprise.

"I don't think they had looked into it," he said. "They didn't have expectations for the paintings."

They are set for a pre-auction tour of Amsterdam and New York, before a final stop in London ahead of the sale.

"What's extraordinary is that the paintings were completely unknown. They had never appeared in any of the Rembrandt literature of the 19th or 20h century," said Pettifer. "They are not grand, formal commissioned paintings, I think they are the smallest portraits that he painted that we know of."

SHARE This Awesome Bit Of History Uncovered With Your Friends…