  Let p:C' -> C be an unramified double covering of irreducible smooth curves
and let P be the attached Prym variety. We prove the schematic theta-dual
equalities in the Prym variety T(C')=V^2 and T(V^2)=C', where V^2 is the
Brill-Noether locus of P associated to p considered by Welters. As an
application we prove a Torelli Theorem analogous to the fact that the g-th
symmetric product of a curve D of genus g determines the curve.
