When an affix declaring the time of an Asterism is elided by lup -- lubaviśeṣe [[4.2.4]], the 7th and the 3rd case-affixes are used after the word, whose affix is also elided.,
Ex. पुष्येण पुष्ये वा पायसमश्नीयात् 'when the moon is in the Asterism of Pushya, let him drink milk.' See Rules 4.2.4 and 4.2.5. So also मघाभिः पललौदनं मघासु पललौदनम्. But not so here पंचालेषु वसति 'he lives in Panchala'. Here the country is meant, and not a star, though here also there is elision of the Taddhita affix.
Why do we say 'lup elision'? Observe मघासु ग्रहः 'the planet in the Magha.' Here there is no elision. But why not in अद्यपुष्यः, अद्यकृत्तिका. Because they do not denote location, which is understood. It is when location is expressed by such words that we may use the third case-affix in the alternative.,