Anatolian languages
This continues a Proto-Indo-European alternation "*suwen-" vs. "*sewol-" (Avestan "xweng" vs. Latin "sōl", Greek "helios", Sanskrit "surya", Welsh "haul", Breton "heol", Old Irish "suil" "eye"), a remnant of an archaic, so-called "heteroclitic", declension pattern that remained productive only in the Anatolian languages.