  For a semisimple real Lie group $G$ with an irreducible representation $\rho$
on a finite-dimensional real vector space $V$, we give a sufficient criterion
on $\rho$ for existence of a group of affine transformations of $V$ whose
linear part is Zariski-dense in $\rho(G)$ and that is free, nonabelian and acts
properly discontinuously on $V$.
