The Alans converted to Byzantine Orthodoxy in the first quarter of the 10th century, during the patriarchate of Nicholas I Mystikos. Al-Mas‘udi reports that they apostasized in 932, but this seems to have been short-lived. The Alans are collectively mentioned as Byzantine-rite Christians in the 13th century. The Caucasian Alans were the ancestors of the modern Ossetians, whose ethnonym derives from the name "Ās" (very probably the ancient "Aorsi"; al-Ma'sudi mentions "al-Arsiyya" as guards among the Khazars, and the Rus' called the Alans "Yasi"), a sister tribe of the Alans. The "Armenian Geography" uses the name "Ashtigor" for the most westerly located Alans, a name which survives as "Digor" and still refers to the western division of the Ossetians. Furthermore, in Ossetian, "Asi" refers to the region around Mount Elbrus, where they probably formerly lived.