Orientability Of Moduli Spaces And Open Gromov Witten Invariants

We show that the local system of orientations on the moduli space of J-holomorphic maps from a bordered Riemann surface is isomorphic to the pull-back of a local system defined on the product of the Lagrangian and its free loop space. The latter is defined using only the first and second Stiefel-Whitney classes of the Lagrangian. In the presence of an anti-symplectic involution, whose fixed locus is a relatively spin Lagrangian, we define open Gromov-Witten type invariants in genus zero.