Second order theta divisors on Pryms

Abstract

Van Geemen and van der Geer, Donagi, Beauville and Debarre proposed characterizations of the locus of jacobians which use the linear system of $2\Theta$-divisors. We give new evidence for these conjectures in the case of Prym varieties.

AMS-Latex, 11 pages, the exposition has been modified, Proposition 5 has been modified