The affix cha comes after the dvanda compound of the names of lunar mansions, when it is in the Instrumental -3rd case in construction, to signify time connected with the asterism whether there be a specification or not.,
Thus राधानुराधीया रात्रिः, तिष्यपुनर्वसवीयमहः ॥ So also when there is no specification : as अद्य राधानुराधीयम्, अद्य तिष्यपुनर्वसवीयम् ॥ The affix here in these two last cases is not elided, though there is no specification of time, because the present sutra, being a subsequent sutra, will debar 4.2.4 which ordains lup 1.4.2.,