After Nadezdha's death, Stalin became increasingly close to his sister-in-law Zhenya Alliluyeva; Montefiore believed that they were probably lovers. There are unproven rumours that from 1934 onward he had a relationship with his housekeeper Valentina Istomina. Stalin had at least two illegitimate children, although he never recognised them as being his. One of them, Konstantin Kuzakov, later taught philosophy at the Leningrad Military Mechanical Institute, but never met his father. The other, Alexander, was the son of Lidia Pereprygia; he was raised as the son of a peasant fisherman and the Soviet authorities made him swear never to reveal that Stalin was his biological father.