Please read Alexey Sklyarenko's annotations on Pale Fire, Ada and other Nabokov works here.
Describing his novel Letters from Terra, Van Veen (the narrator and main character in VN’s novel Ada, 1969) mentions the three scientists one of whom had been kidnapped by a laundryman and transported to Tartary (on Demonia, aka Antiterra, Earth's twin planet on which Ada is set, Tartary occupies the territory of the Soviet Russia):
Ada’s letters breathed, writhed, lived; Van’s Letters from Terra, ‘a philosophical novel,’ showed no sign of life whatsoever.