Editor’s Note: This article was updated with the final sale price and other details following the auction’s conclusion.

London CNN  — 

A pair of previously unknown and “exceptionally rare” portraits by Rembrandt sold for over £11.2 million ($14.2 million) Thursday after they were discovered in a private collection in the UK.

Depicting relatives of the Dutch master, the intimate paintings are the last Rembrandt portraits still in private hands, according to Christie’s auction house, which did not reveal the winning bidder’s identity.

Signed and dated 1635, the eight-inch-high pictures are of an elderly couple related to the painter by marriage: wealthy plumber Jan Willemsz van der Pluym and his wife Jaapgen Carels, who were from a prominent family in the Dutch city of Leiden.

Their son Dominicus van der Pluym was married to Rembrandt’s cousin Cornelia van Suytbroec. The couple had one child, Karel van der Pluym, who is thought to have trained with Rembrandt and included the artist’s only surviving heir, Titus, in his will.

In 1635, the year the portraits were painted, the subjects acquired a garden next to that of Rembrandt’s mother in Leiden.

Ahead of the auction, experts at Christie’s said in a press release that the portraits have a “remarkable, virtually unbroken line of provenance.”

The artworks stayed within the sitters’ family until 1760, a year after the death of the couple’s great-grandson, Marten ten Hove. The portraits then traveled to Warsaw, to the private collection of Count Vincent Potocki, before briefly entering the collection of Baron d’Ivry in Paris in 1820 and then James Murray, 1st Baron Glenlyon.

In June 1824, Murray put the artworks up for sale with Christie’s, where their listing described them as “Rembrandt — very spirited and finely colored.”

Since that sale almost two centuries ago, the paintings remained in Britain in the same family’s private collection and were unknown to experts. The most recent owners have not been named.

Henry Pettifer, international deputy chair of Old Master paintings at Christie’s, told CNN in a telephone interview in May that the discovery was made a couple of years ago, as part of a “routine valuation to look at the contents of a house.”

“The pictures were immediately of terrific interest,” he said, adding that the then-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.”

Pettifer told CNN he had been “incredibly excited” to see the paintings, but “at that stage I didn’t jump to any conclusions.”

Details of the earlier sale at Christie’s got the process rolling, followed by a long period of research at the Rijksmuseum in Amsterdam, where the portraits were investigated and underwent scientific analysis.

“What’s extraordinary is that the paintings were completely unknown. They had never appeared in any of the Rembrandt literature of the 19th or 20th century, so they were completely unknown,” said Pettifer.

The identities of the sitters were only confirmed by researchers at the Rijksmuseum.

The “small, very intimate, very spontaneous” nature of the paintings indicated a close relationship with the artist, Pettifer told CNN.

“They are not grand, formal commissioned paintings,” he said. “I think they are the smallest portraits that he painted that we know of.”

The pictures sold as part of Christie’s “Old Masters Part I” auction, which fetched over £53.9 million ($68.5 million) in London on Thursday. The evening’s biggest sale was a previously unknown work by Flemish painter Michael Sweerts that went for almost £12.6 million ($16 million)