  We construct vector bundles $R^r_\mu$ on a smooth projective curve $X$ having
the property that for all sheaves $E$ of slope $\mu$ and rank $r$ on $X$ we
have an equivalence: $E$ is a semistable vector bundle $\iff$
$Hom(R^r_\mu,E)=0$.
  As a byproduct of our construction we obtain effective bounds on $r$ such
that the linear system $|R \cdot \Theta|$ has base points on the moduli space
$U_X(r,r(g-1))$.
