Gabriele Kliem was engaged to her 'dream man' for seven years. But all along he was a Stasi agent, whose job it was to seduce women into handing over secret documents. Linda Pressly on the Stasi 'romeos' who haunted West Germany during the cold war

Gabriele Kliem remembers everything about the day Frank Dietzel walked into her life. It was a sweltering summer's evening in Bonn in July 1977. As she sat on the banks of the Rhine waiting for a male friend, a tall, blond, blue-eyed man strolled towards her. "He looked like my dream man," she recalls, "and I thought, if I could ever meet such a man I would be so, so happy. I fell in love with him the minute he came towards me." Dietzel introduced himself and told her he was a friend of the man she was waiting for, who was sick. Would she like to come to dinner with him instead? "My first reaction was that I should get up and walk away as fast as I could, because a relationship with a man that good-looking would be disastrous. But I didn't, I just didn't."

Dietzel was, in fact, a Stasi spy. He had been sent by East Germany across the border to Bonn, on a mission to seduce Kliem, a 32-year-old translator and interpreter at the American embassy. He told her he was a physicist working for an international research company committed to world peace; beyond that, he remained vague. Three months later, in October 1977, the couple got engaged. And, over the course of their seven-year relationship, Kliem supplied Dietzel with hundreds of secret documents from the embassy - furthering, or so she thought, his noble aims. It was only in 1991, when she was arrested for espionage, that she discovered her former fiance had been a married, East German Stasi officer, who had been awarded a medal for his "work" with her.

We do not know exactly how many women were duped by Stasi agents during the cold war. But over the course of four decades, around 40 were prosecuted for espionage in the Federal Republic of Germany, as a result of romantic relationships with undercover officers of the German Democratic Republic.

The Stasi "romeo" network was the brainchild of Markus Wolf, the former head of East Germany's foreign intelligence service. "When it began," he writes in his autobiography, "I had no idea of the harvest it would bring." Early on in the cold war, the Russian KGB had perfected the art of sexual blackmail - usually one-off assignations with targeted foreign embassy staff in Moscow. But the Stasi operation was more extensive, with agents assigned to develop long-term relationships with their sources. Women were the principal targets - often secretaries in Bonn's many ministries and other government offices. Wolf believed that just one of these well-placed women could prove infinitely more valuable to the Stasi than five or even 10 male diplomats. They knew everything, and were often also responsible for their bosses' private correspondence.

In the years following the second world war, Bonn was a magnet for ambitious young women from all over West Germany. They hugely outnum bered single men in the capital. Kliem shared a house with other women who worked at the American embassy. "Bonn was a very quiet town," she says. "Translators and secretaries were hardly ever invited to official social functions, and single women were eyed suspiciously by married women. It was almost impossible to find a boyfriend, and if you found one, almost impossible to keep him - the stakes were very high. The competition was enormous."

As a result, many of these women were delighted to be subject to the attentions of seemingly eligible men. Marianne Quoirin, the author of Agentinnen aus Liebe (The Spies Who Did It For Love), a book about Stasi "romeos", sat in on the court cases of a dozen women. She says that a woman pursued by an agent was usually vulnerable in some way. "Perhaps she had been left by her boyfriend, or her mother had recently died, or she didn't have many friends. When the romeo approached her, he already knew everything about her - her likes and dislikes, her history. One woman told me that the agent who approached her knew she was interested in the environment, and after two days he was calling her his 'little herb witch'. So they got to the point pretty quickly."

The Stasi did an impressive amount of groundwork before a woman was approached. Scouts were employed to inform officers who might be ripe for romantic cultivation, and thousands of deutschmarks were paid for a job well done. In Kliem's case, a "tipper" befriended one of her best friends, an American, and found out that she had previously been in love with a blond, blue-eyed maths teacher. Hence, Dietzel's looks, and his claim to be a physicist.

In the course of her research, Quoirin was struck by how ordinary - even physically unattractive - some of the Stasi romeos were. "The women definitely weren't going for good looks. It was the old-fashioned manners ... flowers, wining and dining, and, most importantly, these men listened to women. Men often don't, so that was very attractive. Sex didn't play a major role. It was important for Gabriele and a couple of the other women, but actually, it mattered rarely."

Gerhard Beier was one of the more striking Stasi agents. He worked in West Germany for nearly 20 years before the Berlin Wall came down, and was ideologically committed to the German Democratic Republic. "I was fulfilling my patriotic duty, and it wasn't unpleasant," he says now. He would have four or five "cases" on the go at any one time: "I prioritised those I had the best chance of getting information from, and kept the others on a fairly long lead."

Beier posed as an officer of the Bundesnachrichtendienst (BND), the West German intelligence service. "I always carried my false ID with me, so I had an air of secrecy, and this animated women - they found it interesting. And that, generally, was enough." He would nurture a friendship to the point where it became sexual. "So we established a connection - let's say the post-coital relationship - and then from that arose a feeling of obligation to me." This obligation would embroil a number of women unwittingly in espionage activities.

One of Beier's best sources was the daughter of a senior West German army officer. Beier met her after he placed a false recruitment ad in a national newspaper - a ruse he used more than once to acquire new contacts. On meeting her, and realising she had a useful military background, he moved quickly to get her on side. Once their romantic relationship was established, Beier confessed that the job she had applied for did not in fact exist. Instead, using his cover as a BND agent and appealing to her sense of patriotism, he persuaded her to apply for a job that would be of greater use to her country. She became a typist with the then European Community in Brussels. "While she was there," says Beier, "she was able to set her eyes on an incredibly large number of papers, including secret documents. She would either photocopy or photograph them, and then bring them to me."

Although there were romeos who married their targets - Beier was one of them, and is still married to a former source - the large amount of paperwork needed to marry in West Germany meant an undercover officer risked exposure. But the Stasi were not beyond arranging mock marriages if it ensured the compliance of a useful source. A secretary and former nun refused to have sex with her romeo before they were married; she was obliged with a wedding in a small church in Copenhagen. "A Stasi officer played the priest, and took her confession," says Quoirin, "and later on another officer played her mother-in-law at a small reception."

In 1984, after seven years, Kliem tired of her very long engagement to Dietzel. After waiting most of her 30s, she quickly met someone else and got married. In the course of her engagement, she had seen Dietzel about once a month, and on many occasions given him photographs that she had taken of documents in the Office of Defence Cooperation at the American embassy. In spite of an advertising campaign by the federal government warning women of Stasi tactics, she says that she never asked her lover who else was looking at the material she collected. Too fearful of losing him, she lived instead for the days they spent together, when they shopped endlessly for the life she believed they would one day share. It was only at her trial for espionage in 1996, that Kliem discovered her fiance had given most of these consumer goods to his wife in East Germany. More shocking was the revelation that he had passed all her love letters to the Stasi's psychologists. "So they would sit and read and laugh and analyse and see how they could hurt me some more," she says. "To them I was just a laboratory rat or worse - and to him, I was just a tool." Dietzel died in a car crash in the early 90s, some years before Kliem's trial.

As far as we know, from the cases that came to court, none of the women targeted by the Stasi initially realised that their boyfriends were from the GDR. "I would never have consented to a relationship if I had known he was East German," says Kleim. "My father died in a Russian camp at the end of the war. I grew up in Berlin, and we hated anything Russian or East German. And Frank Dietzel knew that."

Kliem does not think of herself as a traitor; she was driven by love and not politics. The court that tried her in 1996 handed down a suspended sentence and fined her. She was lucky. In the years before the Wall came down, women spent years in jail for similar crimes. She does not blame Dietzel for what happened. "If it hadn't been him, they would have sent someone else. Firstly, I blame Germany - there were two systems, two people with the same language, and you would never know the other was the enemy. And then I blame myself. To wait and long for a man - if I hadn't given him that power over me, he couldn't have done it." Today, she lives alone in the Netherlands, unwilling to stay in a country where she suffered so much heartache. Now approaching 60, she spends much of her time selling clothes at craft fairs and looking after abandoned dogs.

Beier met and fell in love with his West German wife while working as a spy. She appears to have forgiven him. But he did not escape completely unscathed: in the early 1980s he was jailed for two years for espionage. On his release, he started his own computer security company, and continues to run it as a pensioner. He remains unembarrassed about his past, seeing it as the exigency of a very particular era in German history. "I always stuck to the party line and did what I was supposed to do. And it was fun."

· Sleeping with the Enemy: Sexpionage is on BBC Radio 4 on Monday November 22 at 8pm.