  Let X be an irreducible symplectic manifold and L a divisor on X. Assume that
L is isotropic with respect to the Beauville-Bogomolov quadratic form. We
define the rational Lagrangian locus and the movable locus on the universal
deformation space of the pair (X, L). We prove that the rational Lagrangian
locus is empty or coincide with the movable locus.
