Fixed divisors on hyperkähler manifolds

Let $X$ be a hyperkähler manifold, and let $A$ be a nef and big divisor on $X$. We show that the fixed part of the linear system $\mathrm{\mid }A\mathrm{\mid }$ is reduced and as a consequence $\mathrm{\mid }2A\mathrm{\mid }$ is mobile. If $X$ has dimension four we also show that if the fixed part of $\mathrm{\mid }A\mathrm{\mid }$ is not empty, the mobile part induces a (rational) Lagrangian fibration.