  Given any representation V of a complex linear reductive Lie group G_0, we
show that a larger semi-simple Lie group G with g=g_0 + V + V* + ..., exists
precisely when V has a finite number of G_0-orbits. In particular, V admits an
open G_0-orbit. Furthermore, this corresponds to an augmentation of the Dynkin
diagram of g_0.
  The representation theory of g should be useful in describing the geometry of
manifolds with stable forms as studied by Hitchin.
