  In the framework of the Einstein-Maxwell-aether theory we study the
birefringence effect, which can occur in the pp-wave symmetric dynamic aether.
The dynamic aether is considered to be latently birefringent quasi-medium,
which displays this hidden property if and only if the aether motion is
non-uniform, i.e., when the aether flow is characterized by the non-vanishing
expansion, shear, vorticity or acceleration. In accordance with the
dynamo-optical scheme of description of the interaction between electromagnetic
waves and the dynamic aether, we shall model the susceptibility tensors by the
terms linear in the covariant derivative of the aether velocity four-vector.
When the pp-wave modes appear in the dynamic aether, we deal with a
gravitationally induced degeneracy removal with respect to hidden
susceptibility parameters. As a consequence, the phase velocities of
electromagnetic waves possessing orthogonal polarizations do not coincide, thus
displaying the birefringence effect. Two electromagnetic field configurations
are studied in detail: longitudinal and transversal with respect to the aether
pp-wave front. For both cases the solutions are found, which reveal anomalies
in the electromagnetic response on the action of the pp-wave aether mode.
