Sigmatic Aorist

Old Church Slavonic Grammar - Verbs - Sigmatic Aorist
... The sigmatic or s-aorist was formed in the following ways Verbs whose stem ends in b, p, d, t, z, s form this aorist by dropping the final consonant and adding the interfix -s- plus the endings -ъ, -, - -om ... Verbs whose stem ends in r or k form this aorist in the same way as previously mentioned, except that intervocalic sigma (s) changes into x, the same set of endings being suffixed to the interfix ... forms of these verbs are not attested, so the sigmatic aorist forms are used as a replacement ...

Proto-Slavic Language - Grammar - Verbs - Grammatical Categories
... the PIE imperfect was lost or merged with the PIE thematic aorist, and the PIE perfect was lost other than in the stem of the irregular verb *věděti "to know" (from ... The aorist was retained, preserving the PIE thematic and sigmatic aorist types (the former is generally termed the root aorist in Slavic studies), and a new productive aorist ... st that arose when the original athematic endings were attached to the sigmatic *-s- affix.) A new synthetic imperfect was created by attaching a combination of the root and productive ...