The December 9 Russian imperial sale is expected to rake in more than one million Swiss francs (USD 1.1 million, 812,000 euros), with the letters alone seen fetching 60,000-80,000 francs, Hotel Des Ventes (HDV) said in a statement.
The letters, written in Russian, French and English, were purchased by an American captain and journalist deployed in Europe during World War II, "transport the reader right into the heart of Russian Imperial life and political intrigue," it said.
Discovering that a number of the letters had been written by the tsars themselves, he quickly realised he was in possession of a historical treasure, explained Claire Piguet, HDV specialist on Russian imperial family manuscripts.
Most of the letters were addressed to Grand Duchess Olga Nicolaievna, the daughter of Nocolas I and sister of Alexander II, who moved to Stuttgart after marrying a German prince who would become King Charles I of Wurttemberg.
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Through the missives, "intimate secrets and plots of the Russian Court are unveiled as well as the beliefs and political pains of the two emperors," HDV said.
The snaps were rescued by Otto Hofmann, a German Bauhaus artist who was serving with the Nazi forces.
"Acting against martial law and putting his own life at risk, the soldier hid and brought back around 30 images which he considered historical evidence and culturally important for Imperial Russia," the auction house explained.
The family of Hofmann, who managed to hide the shots during several years as a prisoner of war, had decided to sell the pictures to "reveal his gesture to all," HDV said.
The vases, which are expected to fetch between 300,000-500,000 francs, were bought by a Swiss couple in Paris in the 1960s, and have been decorating their home in Geneva ever since.