  Let H\subset\GL(V) be a connected complex reductive group where V is a
finite-dimensional complex vector space. Let v\in V and let G=\{g\in\GL(V)\mid
gHv = Hv\}. Following Ra\"is we say that the orbit Hv is \emph{characteristic
for H} if the identity component of G is H. If H is semisimple, we say that Hv
is \emph{semi-characteristic} for H if the identity component of G is an
extension of H by a torus. We classify the H-orbits which are not
(semi)-characteristic in many cases.
