  The aim of this article is the proof of the following result: Let M be a
connected manifold endowed with a regular Cartan geometry modelled on the
boundary X of the d-dimensional real (resp. complex, resp. quaternionic, resp.
octonionic) hyperbolic space. If the group of automorphisms of M does not act
properly on M, then M is geometrically isomorphic to:
  - X if M is compact.
  - X minus a point in the other cases.
