In Code Geass: Hangyaku no Lelouch episode 11, around 16:00 mark, there is a song that starts to play when Lelouch sees the memories of C.C. Again, in the same episode at 19:50, the same song plays for the second time.
I didn't recall hearing this song before episode 11, and episode 13 at 7:35 as well.
I couldn't find the soundtrack from OSTs. What is it?