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In the 1970s Heidemann bought the yacht Carin II, which had been Hermann Göring's, met his daughter Edda Göring, and began an affair with her which lasted for five years. They entertained on the yacht, and their guests included two Second World War generals, Karl Wolff and Wilhelm Mohnke.
Heidemann came forward with his story of lost diaries written by Adolf Hitler in 1983. He sold the rights to them to the West German magazine Stern for DEM 10,000,000 (then approximately USD $6 million), along with his tale about how they had been hidden in a barn in East Germany for many years. Several experts, including the British Hitler expert Hugh Trevor-Roper, came forward to pronounce the diaries to be authentic. However, they were ultimately found to be forgeries, and Heidemann was arrested, tried, and sentenced in 1985 to four and a half years in prison for fraud, as was Konrad Kujau, who had actually done the forging of the books in Hitler's handwriting, as he had done previously with other fraudulent Hitler documents. In a twist in the tale, in 2002 it was revealed that Heidemann had worked for the Stasi, although he claimed he had been a double agent. In the BBC radio 4 programme The Reunion broadcast on Sunday 7 September 2008, Heidemann vehemently denied that he had ever been a spy for the Stasi.
As of 2008 Heidemann was living in poverty.
"I am healthy," he pronounces. "I am a pensioner and get €350 [£280] a month from the social security office. They also pay the rent, my health insurance and my old-age care insurance." 
- Peter Wyden, The Hitler Virus: the Insidious Legacy of Adolf Hitler (2001, ISBN 1559705329), p. 173
- BBC News, Hitler diaries agent was 'communist spy', 29 July 2002
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