"a" vs "an" before parenthesis: which indefinite article should I use when adding parentheses before the subject?

I want to write a remark in parenthesis between an indefinite article and the rest of the subject. If I didn't write this clarification then using the "a" article would be correct; however for the clarification alone "an" should be used.

Which one of the following examples is correct? Both sound weird to my ears.

This experiment used an (arguably small) set of data.

This experiment used a (arguably small) set of data.