Romanian has accusative and dative clitic pronouns, which normally precede the verb in the order DAT-ACC. There is an exception to this rule: if the verb is accompanied by an auxiliary beginning with a vowel, the feminine form of the 3rd person accusative clitic o appears in enclitic position. We propose an OT-based account which derives the basic clitic order through the ranking of clitic alignment constraints. The exceptional behavior of o is due to the interaction of the clitic alignment constraints with the phonological constraints NoHiatus and Onset.
Align (aux, R, V, L) >> Align (acc, R, V, L), NoHiatus >> Align (dat, R, V, L), Directional Integrity, Onset >> Align (acc, L, V, R), Align (dat, L, V, R)