It began as a Ph.D. thesis and a British curator’s unfashionable interest in Restoration art.

“What’s exciting [about the Restoration] is that it’s a period when women came into their own,” said Julia Marciari-Alexander, a curator at the British Art Center.

This weekend, an exhibition of Restoration portraits called “Painted Ladies” will open at the center, marking a unique collaboration between the museum and London’s National Portrait Gallery. The Restoration court was the court of King Charles II.

The collaboration is, in some ways, a chance marriage between one of the world’s most noted museums and its American cousin. And it is a story of two curators whose interests happened to intersect.

“I was working on 17th century [portraits] and found it a completely unworked period,” said Catharine MacLeod, a curator at the National Portrait Gallery.

When she began to assemble a show of Restoration portraits, she found an ally in Marciari-Alexander, who wrote her thesis on the same subject. Given her expertise and Yale’s extensive holdings of British art, Marciari-Alexander’s collaboration with MacLeod was natural.

Curiously, there had not been a major exhibition of Restoration artwork for 20 years, even though the era is rich in the type of drama that often draws museum visitors.

While his wife Queen Catherine miscarried twice, King Charles II had a succession of mistresses. His sexual practice was public knowledge — a portrait of one of his mistresses, on display in New Haven, shows the woman dressed up as St. Catherine in mocking homage to the Queen she cuckolded.

Charles, who granted handfuls of Dukedoms to his illegitimate children, was not shy about having his mistresses painted with his children. The court’s Principal Painter once even presented a mistress and child as Madonna and Jesus.

“They were audacious but also playful,” said Marciari-Alexander of the period’s portraiture.

While it is nothing new for the National Portrait Gallery to send one of their exhibitions to New Haven, it is something remarkable to see the two galleries working together.

“We have never collaborated so extensively,” Marciari-Alexander said.

The “Painted Ladies” exhibition, which opens this weekend at Yale after a run at the National Portrait Gallery, is the fruit of five years of work between Marciari-Alexander and MacLeod.

“We certainly have enjoyed our collaboration,” said Constance Clement, the acting director of the British Art Center.

Last year, Yale ran the Portrait Gallery’s widely lauded exhibition of the work of John Snowden, which closed this September in New Haven. And in a year’s time, the gallery will put on another show with its British counterpart, called “Romantics & Revolutionaries.”

Whether the kind of close cooperation needed for the “Painted Ladies” exhibition will be convenient in the future remains to be seen.

“It’s not always easy to collaborate on an exhibition around the world,” MacLeod said.

Marciari-Alexander had to travel at least twice a year to London to help acquire paintings for the exhibition, but the benefits for all parties involved are nevertheless significant.

“[The British Art Center] is not very well known in London so it’s nice to promote,” MacLeod said. “Obviously people who work in British art are familiar with it, but outside that circle it’s not well known.”