  Magnetoelectric materials have the interesting property of exhibiting
polarization induced by a magnetic field or magnetization induced by an
electric field. As a consequence, a multitude of effects can be produced by
means of controllable external fields. A method for deriving phase velocities
and its corresponding polarization vectors for light rays in nonlinear optical
materials in a nondispersive regime is here revisited and used to study wave
propagation in a certain class of second-order magnetoelectric media. In
particular, the birefringence effect is theoretically examined and it is shown
that it can be used as a tool to obtain most of the second-order
magnetoelectric coefficients $\beta_{ijk}$ of a material having an isotropic
linear sector. Estimates of the effect are presented. Some optical properties
of nonlinear materials presenting a natural optic axis are also discussed.
