RER B :
Please note that because of work taking place at the ORSAY-VILLE RER-B station, it is impossible to go to IAS and LAL through the usual tunnel beneath the tracks. You have to walk back in the direction of Paris 100 meters or so. to the vehicle underpass, and follow the blue signs and arrows that indicate "Facultés Sciences".
Note that it will take an extra 7 minutes walking time.
Another tip: when you come from Paris, select the rear carriage; as in the past, you will be closer to the exit.