Scientists using infrared technology have discovered a wife swap in a medieval manuscript, with a French duke’s first wife painted over with his second when he remarried.

The 15th century manuscript is a book of hours, an illustrated prayer book commissioned by the wealthy, and is called The Hours Of Isabella Stuart.

It is based on the monastic day for use in daily life.

Staff at Cambridge’s Fitzwilliam Museum “noticed there was something slightly odd” about the book so it was examined in a lab, said co-curator Dr Suzanne Reynolds.

She said a “darker area” was noticed on a page “so it was decided to use infrared and see what was going on there”.

“That’s when the under-drawing was revealed,” said Dr Reynolds.

The manuscript was commissioned in 1431 by the mother of the first wife of Duke Francis I of Brittany.

It was a wedding gift to the duke’s first wife the Duchess of Anjou, Yolande of Aragon, that year. She died in 1440.

The duke remarried in 1442 and he had the manuscript altered, with his first wife painted over with his new wife Isabella Stuart.

Yolande, kneeling before the Virgin Mary, was covered up with his new wife and Isabella’s coat of arms was also painted into each corner of the decorated borders.

Scientists discovered that Breton artists working in Nantes had adapted and added illuminations for Isabella, and later, her daughter Margaret.

Variations in the under-drawings helped to distinguish between the original artists who were based in Angers, but the use of different pigments confirmed the modifications and additions were done in Nantes.

Dr Reynolds said: “It’s a very exciting discovery.

“These books in a way are sort of archaeological sites and when you start to uncover what lies under these images it actually unlocks the human story of how these books were commissioned and then passed from one person to another as the story of these different marriages and different dynastic alliances evolved.”

She described the over-painting as “not unique but unusual”.

“What they also did was add in the coats of arms of the second wife into the borders around the manuscript,” she said.

“That’s not so unusual, that happened quite a lot in manuscripts if they passed from one owner to another.

“The new person might add their coat of arms into the manuscript to demonstrate their ownership of it.”

The discovery “fleshed out the story” of the manuscript, she said.

“We knew it had been commissioned by the mother of the first wife, Yolande,” said Dr Reynolds. “She’s featured later in the manuscript.

“We knew that somehow it ended up with Isabella Stuart because it’s her coats of arms.

“The connection between those two is Duke Francis.

“I suppose what we didn’t know is how the manuscript had been reworked in order to make it Isabella’s own.”

The Hours Of Isabella Stuart is an “absolute masterpiece of illumination”, she said, adding that it was one of the most extensively decorated books of hours in existence.

The manuscript was given to the museum by its founder Richard Fitzwilliam upon his death in 1816.

It will go on show in the Fitzwilliam Museum’s new exhibition The Human Touch which opens on Tuesday May 18.