Click pictures to enlarge them.
Buy tickets at ATM (by 10 it's much cheaper)... ...because counter is often closed! (Here, Denfert-Rocheau)
RER B Denfert-Rochereau platform It is better to ride at the train front outdoor
check at the display that the train stops at Massy-Palaiseau (this one is not) ... or Lozere (Ecole Polytechnique) if you want to climb the hill (this one does both)
TV displays indicate trains bound for Massy-Palaiseau and Saint-Remy (Ecole Polytechnique) Get off the train at Massy-Palaiseau at the front of the train, climb the staircase
Turn slightly to the left to exit the corridor and... ... go down the staircases to go the the 91.06 bus stop
Wait in front of the 91.06 bus stop... ...You need to use another ticket (Buses always stop at Ecole Polytechnique)

All images (c) Frank Nielsen, November 2008.