On two theorems about local automorphisms of geometric structures

This article investigates a few questions about orbits of local automorphisms in manifolds endowed with rigid geometric structures. We give suf- ficient conditions for local homogeneity in a broad class of such structures, namely Cartan geometries, extending a classical result of Singer about locally homogeneous Riemannian manifolds. We also revisit a strong result of Gromov which describes the structure of the orbits of local automorphisms of manifolds endowed with A- rigid structures, and give a statement and a simpler proof of this result in the setting of Cartan geometries.