  Let X be a smooth projective curve of genus g>1 defined over an algebraically
closed field k of characteristic p>0. Let M_X(r) be the moduli space of
semi-stable rank r vector bundles with fixed trivial determinant. The relative
Frobenius map F : X \to X_1 induces by pull-back a rational map V: M_{X_1}(r)
\to M_X(r). We determine the equations of V in the following two cases (1)
(g,r,p) = (2,2,2) and X non-ordinary with Hasse-Witt invariant equal to 1 (see
math.AG/0005044 for the case X ordinary), and (2) (g,r,p) = (2,2,3). We also
show, for any triple (g,r,p), the existence of base points of V, i.e.,
semi-stable bundles E such that F^* E is not semi-stable.
