I have a question about the music that accompanies the last two minutes of the mirror pas de deux. Does anyone know which specific piece it comes from? This theme is subsequently reprised during the final pas de deux, for about one minute---in the midst of "Francesca da Rimini"---just before Tatiana returns Onegin's letter. It sounds very familiar, but I'm unable to recall where it is from. This has been driving me insane for the last few days.
According to this page, it's a duet scene from Tchaikovsky's "Romeo & Juliet":
The most often employed passages come from The Seasons as well as the opera Cherevichki (The Tsarina’s Slippers, 1885). A duet from Tchaikovsky’s Romeo and Juliet serves as basis for Tatiana and Onegin’s Act I Pas de Deux. The second movement of Francesca da Rimini can be heard in the Act III Pas de Deux.
Apparently it is rarely performed and was completed by Taneyev after Tchaikovsky's death: