It was a marriage with three people in it. A wife, her husband and his female alter ego. In 1930 the Danish artist Einar Wegener became the first man to have a sex-change operation, after he had been married for 15 years to Gerda, another painter. With her support and even enthusiasm, Wegener went to Germany and a hospital in Dresden for a series of operations that changed him into Lili.

Wegener's story was forgotten about, partly because of the chaos in Germany in the 1930s and 1940s. It has become accepted that the "first" sex change was carried out in 1952 on an American GI, George Jorgenson, who became Christine. She died in 1989, feted as the pioneer who led the way for others such as the British writer James Morris (who became Jan).

The novelist David Ebershoff came across a tiny reference to Einar and his story in a book picked up by chance in a secondhand bookshop in New York. "I was struck by the mention that he was happily married."

Ebershoff, a senior editor at Random House, then started to do some research. He had no real leads and knew almost nothing about transsexuals. "I had never knowingly met anyBody who had had the operation." With little more information on Einar to come by in America, he nevertheless had a "gut instinct" that the story could make a fine book.

In 1997, Ebershoff went to Denmark. He scoured old newspapers on microfiche from the late 1920s to 1931 and found a few references to the story. Then he learnt of a diary that Lili had written in the last months of her life. It had been privately and posthumously published and one copy had been lodged in the Royal Library in Copenhagen.

"I gradually built up a picture of the two of them ? or really the three of them, because the Einar-Lili character was very much there as well." Yet the diary struck Ebershoff as "odd". He said: "There was no sense of fear or foreboding. Everything in there was seen as being for the good."

The diary also mentions the German doctor, who was found by Gerda and regarded Einar's case as a physical not a mental problem. Ebershoff felt he had to visit Dresden.

"I spent five days there tracing Einar's footsteps as, much as I could. Of course the city that he knew and came to love is no longer there." But Ebershoff did stumble upon "a creaky institution", the Dresden Hygiene Museum, founded in the 1930s, which housed a small library on the medical history of the city.

"There I came across a limited amount of information about Einar as well as old photos of the women's clinic where he had the three operations needed to change him into a woman. It helped me describe the rooms in my own book as well as the types of medical equipment and procedures used at the time."

Back in Denmark, Ebershoff found some paintings that Gerda had done of Einar. One story, which may be apocryphal, had Gerda asking Einar to pose for her as a woman wearing stockings because her artist's model had not shown up. This experience may have spurred Einar to find his true self. According to Ebershoff, Gerda's paintings of Lili portray her as "girl-like and coquettish. Much younger looking than Lili actually was".

In many ways Ebershoff was more intrigued by Gerda than by Einar. "After all, she helped him through it all. It was Gerda who found out about the hospital in Germany. I came to the conclusion that she was intellectually much more of a free spirit, who better understood what needed to happen to her husband."

After researching their story on and off for 18 months, Ebershoff realised it would be better as a novel, The Danish Girl (it will be published by Weidenfeld & Nicolson this month). I just thought it would work better that way. I could examine more easily their relationship and their marriage."

The Danish Girl has a happy ending, but Lili's own story ended in tragedy. After the first operation, Gerda and Lili drifted apart. Lili fell in love with an art dealer and thought she might somehow be able to have children by him. So she went for another operation in Dresden. But it went horribly wrong. She died a few weeks later, having never left the hospital.

"I'm sure the hospital was sincerely trying to help Lili," says Ebershoff. "But I also think it was part of a social engineering experiment. It was a false hope. She had been led on."

After Lili's death, Gerda married an Italian army officer and moved to Paris where, for a while at least, her reputation as an artist flourished. She died in the late 1930s.

With The Danish Girl, which has just been published to critical acclaim in America, Ebershoff has written what he calls a romance and a love story. I was not interested in writing about the details of sex change. It was the emotions that fascinated me."

© Richard Brooks Sunday Times 5/3/2000