Please read Alexey Sklyarenko's annotations on Pale Fire, Ada and other Nabokov works here.
After the dinner in ‘Ursus’ with Ada and Lucette (Van’s and Ada’s half-sister), Van Veen (the narrator and main character in VN’s novel Ada, 1969) asks Lucette to tell him the name of Ada’s fiancé and promises to reward her with a very special kiss:
‘My dear,’ said Van, ‘do help me. She told me about her Valentian estanciero but now the name escapes me and I hate bothering her.’