http://fmg.ac/Projects/MedLands/RUSSIA,%20Rurik.htm

ESFIR, daughter of ---. She is named as Sviatoslav I´s mistress in Europäische Stammtafeln. The primary source which confirms her parentage and relationship with Sviatoslav has not yet been identified.

Grand Prince Sviatoslav had one illegitimate child by Esfir, Oleg Sviatoslavich.