Police later found other body parts further downstream and in Sokolov’s flat.
He is said to have planned to get rid of the body before publicly taking his own life while dressed as Napoleon.
Sokolov had been living with Ms Yeshchenko for at least three years. He organised Napoleonic re-enactments – in which he played the part of Napoleon and she also took part.
He wrote dozens of historical research papers, some of them co-authored with Ms Yeshchenko.
She had moved from the southern Russian region of Krasnodar to St Petersburg and was a postgraduate student at the time of her death.
According to students quoted by AFP, Sokolov enjoyed speaking French and did impressions of Napoleon. They said he called Ms Yeshchenko “Josephine”, after Napoleon’s consort, and asked to be addressed as “Sire”.
In court Sokolov alleged that Ms Yeshchenko had attacked him with a knife during a blazing row. It was then that he shot her.
Russian media say he also blamed persecution by an academic rival for his actions.
Ms Yeshchenko’s parents rejected his version of events.