vowel contraction after "προ-" preverb in Ancient Greek

Question

Like περι-, προ- preverb keeps its final vowel when added to a radical as in "προ-αιρέω". But my French->Ancient Greek dictionary, the old Bailly, tells me that προβάλλω becomes either προέβαλον either προῦβαλον at the aorist 2. I can't find any further explanations in my grammar books : why such a contraction ?

Can someone give me an explanation about these two forms ? Do the vowels contractions after προ- depend on the phonetic environment or is it something linked to language evolution ?

NB : same remark with προκόπτω > imperfect προύκοπτον in this extract but > προέκοπτον in Galatians 1.14 .

Answer 1

Contraction of προ- with the augment is optional: both προέβαλον and προὔβαλον (with coronis and acute, not προῦβαλον with circumflex) occur, although in Attic the contraction happens much more often than not. ου is the regular result of the contraction of ο+ε in Attic (and in some other dialects); however, προ only contracts with a following ε when that ε is the augment, so not in e.g. προελθών.