The affix ḍhak comes in the sense of a descendant after the word bhrū and the augment vuk is added when this affix is to be applied.,
Thus भ्रू + ढक् = भ्रू + वुक् + ढक् (I. I. 46) = भ्रूव् + ढक् = भ्रौवेयः (VII. I. 2),
भ्रवो वुक् च
Adhyāya 4 · Pāda 1 · Rule 125