Cologne- Seven hand-written letters in which Czar Alexander II of Russia and his mistress, Princess Ekaterina Dolgorukaya, explicitly describe sex are to be auctioned this Friday in the German city of Cologne.

A spokesman for the Venator & Hanstein auction house said each of the letters, which range in length from 4 to 13 pages, was expected to sell for between 13,000 and 19,000 euros (17,320 and 25,320 dollars).

The monarch and his mistress exchanged the letters in French or Russian between 1869 and 1871.

"I'm expecting us to make love at least three times tomorrow," is one of the princess's less lurid sentences in the letters, an auctions spokeswoman said.

The letters are being offered by a French aristocratic owner and are only a "small part" of the exchange.

Venator & Hanstein said the bulk of the letters were in the State Archives in Moscow.

The czar's relationship with Dolgorukaya lasted 14 years and was an open secret at court. They wrote to one another almost every day and frequently referred to their sexual practices in the palace.

After Alexander's consort died in 1880, the czar quickly married "Katya." The union was of short duration, as he was assassinated.