You may have your hotel in Paris, in this case, you'll have to take RER B (see location). It's better to choose your hotel near RER B line.
Hotels around Saclay (updated)
Residence Hotels around Saclay
Hotels in Paris (updated) BE CARREFUL: Reservations in APPOLLON HOTEL only available until Wednesday septembre 24th !!!!!
Youth hostels in Paris
Paris Tourism (you can book your hotel in Paris)