  We discuss the possibility of exploring an unbroken U(1) gauge interaction in
the dark sector by means of gravitational waves. Dark sector states charged
under the dark force can give a macroscopic charge to astronomical bodies. Yet
the requirement of having gravitationally bounded stars limits this charge to
negligible values if the force has a sizeable strength. Gravitational tests are
only possible if the dark force is weaker than gravity. By solving the
Einstein-Maxwell field equations, we study in detail an explicit model for dark
charge generation and separation in a neutron star. Charged states originate
from the decay of neutrons inside the star into three dark fermions; we show
that in this model the equation of state is consistent with limits on neutron
star masses and tidal deformability. We find that while the dark force can be
observed in binary mergers (making them an optimal observational test even
though with limited precision), it is Debye screened in binary pulsars (for
which more precise data exist). The emitted radiation in the inspiral phase of
a binary system is modified and the dark force tested at the level of the
uncertainty of the experimental detection. The test covers a region where
current limits on deviations from Newton inverse-squared law come from
geophysical and laser-ranging observations.
