रात्रेश्चाजसौ

The affix ङीप् comes after the word रात्रि in the छन्दस् (Vedas) and in denoting a Name except the affix जस् (1st - Nominative Case Plural) is added.
Thus या रात्री सृष्टा, रात्रीभिः (3rd pl); (Rigveda X. 127. I.I.35.1) but in the nominative plural we have रात्रयः, the regular plural of the word रात्रि, as in the sentence यास्ता रात्रयः, instead of रात्र्यः ॥

Vart:- According to Katyayana, nip is not added, not only when we apply the nominative plural termination जस्; but in all other terminations beginning with जस् also. Thus रात्रिं सहोषित्वा; here in the accusative singular case also the ङीप् is not employed.

But how do you explain the form रात्र्यः in the following तिमिरपटलैरवगुण्ठिताश्च रात्र्यः; This 'ratrya' is the nominative plural of 'ratri' which is formed by the feminine affix nip; and not by nish. The forms evolved by adding ङीष् or ङीप् are one and the same, except with regard to accent; that formed by nish has udatta accent on the final: the other has it on the initial. The word रात्रि is formed by the krit affix त्रिप् (Un. IV. 67), and therefore, it is a word which is governed by 4.1.45 because it is a word falling in Bahuvadi class, by virtue of the general subrule "a word ending with the vowel इ of a krit affix, other than क्तिन् belongs to Bahuvadi class; some say that every word ending in इ, if it has not the force of the affix क्तिन् belongs to this class". Therefore it takes ङीष् in forming the feminine.