Es with diaeresis

Es with diaeresis (С̈ с̈; italics: С̈ с̈ ) is an additional letter of the Cyrillic script which was used in the Bashkir alphabet of Nikolai Katanov. It is composed of the letter es $\langleС\rangle$ with a diaeresis. It was notably used in a Bashkir translation of the gospel by the Bible society published in 1902. It was transliterated using the letter the $\langleҪ\rangle$ in the Bashkir Cyrillic alphabet of 1938.

Computing codes
Being a relatively recent letter, not present in any legacy 8-bit Cyrillic encoding, the letter С̈ is not represented directly by a precomposed character in Unicode either; it has to be composed as С+̈|◌̈ (U+0308).