Hi,
I was expecting responses from elsewhere, but try
http://www.ukbutterflies.co.uk/links.php as a start, especially imho Guy's, Roger's and Matt's pages... they give some idea of the species you may find at that time of the year, though not specifically for your locality.
I spend time each year in Samoens, not far as the crow flies, but have never been there in September.... if I were, I personally would be hoping to see Tree Grayling, Damon Blue, Water Ringlet among others.
As has been said before on this site, you are unlikely to be disappointed, compared to the UK at least, if you seek out spots that look good for butterflies, you never know what you will find!
When you have been, and if you take photos, post some on this overseas thread, there are a number of us that enjoy seeing what you have found.