Grammatical Sūtra: छन्दसि पुनर्वस्वोरेकवचनम् chandasi punarvasvorekavacanam 
Individual Word Components: chandasi punarvasvoḥ ekavacanam
Sūtra with anuvṛtti words: chandasi punarvasvoḥ ekavacanam anyatarasyām (1.2.58), dvayoḥ (1.2.59), nakṣatre (1.2.60)
Type of Rule: atideśa
Description:
In the Vedas, the two stars, Punarvasû, may optionally be singular, (and connote a dual). Source: Aṣṭādhyāyī 2.0
In the domain of Chándas (= Veda) the singular (ekavacaná) of the nominal stem púnarvasu- `n.pr. of an asterism consisting of twin stars' is used [for the dual optionally 58-59]. Source: From Aṣṭādhyāyī of Pāṇini In Roman Transliteration translated by Sumitra M. Katre, Copyright © 1987. Courtesy of the University of Texas Press.