As observed by Hyllested and Cohen (2007), no inherited Greek lexeme contains an initial sequence εὐ-, αὐ-, οὐ- followed by a labial consonant; this leads the authors to propose a sound change which operated in the early stages of Greek: *Vu̯ > ŭ /#C+lab, which explains forms such as ὑφαίνω ‘weave’ for expected †αὐφαίνω ū /CC+lab. After this, I gather additional possible examples of this rule and discuss each instance in detail. The evidence is found to strongly support the proposed rule.
Tore Rovs Kristoffersen (Fri,) studied this question.