Originally Posted by
BigE
The nearby Porte D'Orleans stop is inside the Paris zone, but it's not as convenient (unless you happen to already be near there). I'm not sure about the newer stop that tbm13 mentioned.
The t+ ticket works fine for the Mairie de Montrouge stop. My understanding is that the metro is not divided into zones whereas the RER is. The t+ ticket is valid for all metro stations but only for RER stations in zone 1. Laplace is in zone 2.