Please read Alexey Sklyarenko's annotations on Pale Fire, Ada and other Nabokov works here.
In the conversation with Van before the family dinner in “Ardis the Second” Demon Van (in VN’s novel Ada, 1969, Van’s and Ada’s father) twice repeats the word “capital:”
‘I don’t know if you know,’ said Van, resuming his perch on the fat arm of his father’s chair. ‘Uncle Dan will be here with the lawyer and Lucette only after dinner.’
‘Capital,’ said Demon.
‘Marina and Ada should be down in a minute — ce sera un dîner à quatre.’