Supelec is reachable with the line B of the RER (same line as the airports Orly and Roissy Charles De Gaulle). You have to go to the RER B station called "Le Guichet". Once there, you have to get a shuttle that takes you to Supelec.
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PLEASE BE CAREFUL, not all the RER B trains will stop at "Le Guichet". You have to look at the train name or the information screen on the station platform. Here is the time table of the RER B :
Trains called PLAN and PEPE stop at "Le Guichet".
Here is the Paris metro map:
We propose some hotels near the RER station Denfert-Rochereau: