oh, they have an open air market on rue St CHarles itself on Tuesdays and Fridays (and the street does tend to have food shops on it, anyway), also, it runs south of CH M metro. And there is one under the metro centered around the Motte Picquet metro stop that is pretty good. It's on Sundays and Wednesdays. Convention is Tu, Th and Sunday, that's right. Lots of markets in that area because it is so residential.

Thanks Fodorites!!!! lots of useful information; as we are arriving on saturday(september 15) we will be interested in markets from sunday to wednesday; leaving Paris on thursday.
Once again thanks to all and keep sending more tips!!