Śaiśirīya Śikṣā 26

26

o au uvarṇa ityeṣāmanusvārapavargayoḥ
virāme ca vivṛttau ca madhye ca vyañjanadvaye

of these: o, au, and the u-vowel; before an anusvāra and the pa-class; / and at a pause, and in hiatus, and in the middle between two consonants,