It can take a very long time to obtain Carte de Sejour in France (even if officially it shouldn't) which is a problem for some students, researchers and other people who need to travel often. Therefore, I suggest to visit Prefectures which have a special department dealing with scientists and a separate queue, so it is usually much faster to get Carte de Sejour. For example, Prefecture Evry has one and the stuff dealing with scientists is polite and helpful. I also strongly advise you to avoid Nanterre Prefecture and Antony Prefecture, if possible - it took 6 months to get Carte de Sejour for a few of our unlucky foreign students. Also, don't forget to get support letters from your french advisor and the head of the department saying that you need Carte de Sejour quickly. Remember that it is possible to get Carte de Sejour reasonably fast, just not everyone wants to bother with you application ... Good luck!
