Her eyes are warm and trusting and her lips curl upwards to form the beginning of a proud smile. Marianne, barely 14, was plainly flattered to be put in charge of her baby cousin Gerhard.

The snapshot was taken in 1932. A few years later she was locked up in a psychiatric ward, forcibly sterilised and, by 1945, starved to death by Nazi doctors. Baby Gerhard went on to become one of the world’s best-paid artists, Gerhard Richter — who made an iconic painting out of the family photograph.

Next week in London that painting, Tante Marianne, is expected to reach £2 million in a Sotheby’s auction — not only because it is a powerful work of art, but because it encapsulates Germany’s tangled history, its sense of guilt and victimhood.

The successful bidder will be buying part of a complex puzzle that is only now being unravelled by the writer Jürgen Schreiber, whose book A Painter from Germany digs into the private and public lives of Herr Richter.

“Richter has given a face to the 250,000 people who were killed as part of the Nazi euthanasia programme,” Herr Schreiber says. The Nazi plan to kill the mentally ill in pursuit of racial perfection is rarely discussed, even today. Herr Richter painted his young aunt in 1965, knowing that she had died terribly but without being aware of the details.

These he discovered only when Herr Schreiber visited him with clear proof that Marianne had been dumped in a mass grave in February 1945.

But there was another shock for the artist, 74, who now lives in Cologne. Herr Richter’s late father-in-law, Heinrich Eufinger, was the SS doctor responsible for carrying out the sterilisation of the mentally ill and for implementing the euthanasia programme. The artist, without realising it, had married into a family deeply implicated in the killing of his beloved aunt.

The painter was stunned. He had been a young boy in Dresden when it was bombed by the Allies in 1945 and war themes had always been part of his repertoire: a picture of Hitler, of Stuka bombers, of his uncle in uniform. One of his favourite subjects was his father-in-law.

His work Family at the Seaside shows the former SS officer in swimming trunks, together with his daughter, who would later become Herr Richter’s wife. “Eufinger had told him that he was an honorary member of the SS, but we found out that he was far, far more — a believer, an early Nazi member always ready to help his SS comrades.”

Perhaps Herr Richter knew more, speculates Herr Schreiber, but chose not to dig deeper. The artist no longer wants to discuss it. “I can’t stand the way so much emotional significance has been loaded on to the picture of Tante Marianne,” Herr Richter told The Times. “Schreiber’s book is stupid and sensational. I certainly won’t be attending the auction. It would hurt too much.”

Some German galleries are scrambling to see whether they can raise enough cash to bid in Wednesday’s auction.

The painting, which is being offered by its present owner, Gisela Gross, 87, of Stuttgart, may gain in value from its painful subject. “Pain makes a painting more expensive,” Herr Schreiber says. The artist will not benefit directly but, if the bidding goes high, Herr Richter’s other paintings will gain value.

It is a ghostly picture, like a slightly blurred photograph. Only now is it becoming clear that the ghosts haunt the artist. How much did he really know about his father-in-law’s wartime activities? Eufinger spent some years as a Russian prisoner of war before becoming a leading doctor in the East German communist state, his Nazi past apparently buried or forgotten. Herr Richter belongs to the German generation that was confronted with stony silence as soon as it asked questions of its elders. If he had asked, if he had been given honest answers, would he have discovered the man who signed away the life of his aunt? “This painting,” Herr Schreiber says, “tells a very German story.”

THE RICHTER SCALE

• February 1932 Gerhard Richter born in Dresden
• February 1945 Aunt Marianne is murdered
• 1951-57 art student Richter meets and marries Ema Eufinger
• 1961 moves to West Germany
• 1989 controversial cycle of paintings depicting Baader- Meinhoff terrorists
• 2001 Three Candles sells for $3.9million
• 2004 Richter’s annual turnover is $120m (£65m)

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