hebrew - Why "מִי וָמִי" instead of "מִי וּמִי"?

I have always thought that as a general rule, the vav conjunctive prefix is usually a "וְ". But, when the word being "prefixed" either starts with a shva or starts with a "lip-letter" (בומ"פ), the vav changes to a "וּ" (kubutz).

However, I have come across a few of exceptions to the above rule, only one of which I actually remember at the moment. (Other examples are welcome as comments or as part of an answer.)

The exception I can think of right now is Exod. 10:8: "מִי וָמִי הַהֹלְכִים", in which I would think it should be "מִי וּמִי הַהֹלְכִים".

Why is it not "מִי וּמִי"? Is my above rule wrong or missing something, or is this just a particular exception, and if so, why?

Answer

According to Gesenius, you are mostly right. Usually, a vov will take a shuruk when preceding a labial consonant, as well as when preceding a simple shva. The exception seems to be when the vov is right before the accent, and the word is at the end of a clause, sentence, or group of nouns.

From Wikisource:

(e) Immediately before the tone-syllable [the Waw] frequently takes Qameṣ, like בְּ‎, כְּ‍‎, לְ‎ (see §102f), but in most cases only at the end of a sentence or clause (but cf. also וָכֹא‎ 1 K 22³⁰), e.g. וָמֵ֖ת‎ Ex 21¹² (on the other hand, in verse 20 וּמֵ֖תִ‎ is in closer logical connexion with what follows); 2 K 7⁴ וָמַ֣הְנוּ שָׁ֔ם‎, וָמַ֑תָנוּ‎ and וָמָֽתְנוּ‎; Ru 3³ וָסַֹכְתְּ‎; [Ps] 10¹⁵ וָ֝רָֹע‎; 1 S 9⁴ וָאַ֔יִן‎; 2 S 13²⁶ וָלֹ֔א‎; Ez 47⁹ וָחָ֔י‎; cf. also (with Ṭiphḥa) Gn 33¹³, 2 S 15¹². The very frequent connexion of nouns expressing kindred ideas, by means of וָ‎, is due simply to considerations of rhythm, for even in such cases the Wāw must immediately precede the tone-syllable, which must be marked by a disjunctive accent, e.g. תֹּ֫הוּ וָבֹ֫הוּ‎ Gn 1², יוֹם וָלַ֫יְלָה‎ Gn 8²² (see also the previous examples); Gn 13¹⁴ (thrice); Ex 25³ זָהָב נָכֶ֫סֶף‎; [Ps] 96⁷ כָּבוֹד וָעֹז‎; [Ps] 76⁷ וְרֶ֫כֶב וָסוּס‎; Gn 7¹³ וְשֵׁם־וְחָם וָיֶ֫פֶת‎; נֹחַ‎ 1 K 21¹⁰ כֹּה וָכֹה ;אֱלֹהִים וָמֶ֑לֶךְ‎ thus and thus; Est 1⁸ אִישׁ־וָאִֽישׁ‎ at the end of the verse, but in [Ps] 87⁵ אִישׁ וְאִישׁ‎ in spite of the D^eḥi with the second אִישׁ‎, because it is closely connected with the following predicate. Also with three words פַּ֫חַד וָפַ֫חַת וָפָ֑ה‎ Is 24¹⁷. [...]