The Scorza correspondence in genus 3

In this note we prove the genus 3 case of a conjecture of Farkas and Verra on the limit of the Scorza correspondence for curves with a theta-null. Specifically, we show that the limit of the Scorza correspondence for a hyperelliptic genus 3 curve C is the union of the curve ${\{x, \sigma(x) \mid x \in C\}}$ (where σ is the hyperelliptic involution), and twice the diagonal. Our proof uses the geometry of the subsystem Γ₀₀ of the linear system |2Θ|, and Riemann identities for theta constants.