Duke Constantine Petrovich of Oldenburg
Konstantin Friedrich Peter von HolsteinGottorp, Duke of Oldenburg was a son of Duke Peter Georgievich of Oldenburg and his wife Princess Therese of NassauWeilburg Known in the court of Tsar Nicholas II as Duke Constantine Petrovich of Oldenburg, he was the father of the Russian Counts and Countesses von Zarnekau.
They had two children. Their second son, Duke Peter Georgievich of Oldenburg, the father of Constantine Petrovich, was born in 1812.
Source: Wikipedia