The Sooty Copper is a small butterfly that is easily overlooked.
It is in the family commonly known as the blues, which in Britain includes the Common Blue, Holly Blue and the Small Copper, all of which most reasonably competent naturalists will be familiar with.
This one is not found in the UK but is quite common in France, There are many more species of Butterflies to be found in France than in the UK, partly because it has a bigger range of habitats. Mediterranean, Alpine, through to northern European like us. There is an excellent web site to help you identify any strange butterflies you may come across called Butterflies of France.