(The Hill) – Two “exceptionally rare” paintings by the renowned Dutch artist Rembrandt Van Rijn have been discovered in a private collection, and are now heading to auction where they’re expected to fetch between $6 and $10 million.

Henry Pettifer, of the British auction house Christie’s London, discovered the oil paintings while conducting a routine evaluation of a family’s collection. The portraits, depicting a husband and wife from the Netherlands, date back to 1635, notes the Financial Times.

Pettifer said the works are previously unknown to art scholars and historians.

“It was extraordinary to me that the pictures had never been studied before,” Pettifer told the Times. “They were completely absent from the Rembrandt literature.”

The family had inherited the artwork that was last purchased in 1824, and didn’t realize the works were originals, notes the Washington Post.

“The family liked the pictures but were never certain that they were by Rembrandt and never really looked into that,” Pettifer told the outlet.

He added, “They have been quietly sitting in this collection, effectively hidden away from any attention at all.”

The works were authenticated by Christie’s and Amsterdam’s Rijksmuseum, and will be auctioned this July, where Pettifer anticipates they’ll fetch upwards of $6 million.