  In this note we prove the following three algebraic facts which have
applications in the theory of holonomy groups and homogeneous spaces: Any
irreducibly acting connected subgroup $G \subset Gl(n,\rr)$ is closed.
Moreover, if $G$ admits an invariant bilinear form of Lorentzian signature, $G$
is maximal, i.e. it is conjugated to $SO(1,n-1)_0$. Finally we calculate the
vector space of $G$-invariant symmetric bilinear forms, show that it is at most
3-dimensional, and determine the maximal stabilizers for each dimension.
