Please read Alexey Sklyarenko's annotations on Pale Fire, Ada and other Nabokov works here.
According to Kinbote (in VN’s novel Pale Fire, 1962, Shade’s mad commentator who imagines that he is Charles the Beloved, the last self-exiled king of Zembla), in a conversation with him Shade mentioned those joint authors of genius Ilf and Petrov: