I've waited a bit, to see if any of the French would throw out an answer.

Likely you heard of this from someone who's read Graham Robb's excellent The Discovery of France, which has a chapter on the cagots. Once I had, I asked French friends and learned that they knew little, if anything: it was another odd, disappeared thing from the once-diverse French cultural landscape, like the stilt-walking shepherds of Landes. For historians, it's not an easy topic- the sources are legends, stories, some architectural features like special doors for them in churches and possible carvings. Despite notions of them having special features like webbed feet, though, they really don't seem to have had any distinguishing characteristics-no special language, racial features. Although they were found in the southwest, mostly, they weren't the obvious possibilities of Basques, Moors, Jews, or Gypsies... you were identified as a cagot simply because your parents were. This meant, of course, that once there were factories, if you left the village and moved to Lille, where no one knew you, you could stop being a cagot. There was also a general effort in the 19th century in France to get rid of embarrassing regional cultures ( they tried hard to get people to stop speaking Occitan and Breton) as well, and between those the cagots could fade unnoticed into regular French life.

The lack of hard facts hasn't prevented speculation, and France has never lacked for armchair anthropologists willing to display their imaginative powers. If you check out the French Wikipedia article on Cagots, you'll find a lot of it displayed, but also what little hard evidence exists and a good reference list ( although French ones) at the bottom.

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