  We discuss the conditions under which plane electromagnetic and gravitational
waves can be amplified by a rotating black hole due to superradiant scattering.
We show, in particular, that amplification can occur for low-frequency waves
with an incidence angle parametrically close to 0 (or $\pi$) with respect to
the black hole spin axis and with a parametrically small left (or right)
polarization. This is the case of the radiation emitted by a spinning
electric/magnetic dipole or gravitational quadrupole orbiting a black hole
companion at large radius and co-rotating with the latter. This can yield
observable effects of superradiance, for example, in neutron star-stellar mass
black hole binaries, as well as in triple systems composed by a compact binary
orbiting a central supermassive black hole. Due to superradiance, the total
source luminosity in these systems exhibits a characteristic orbital modulation
that may lead to significant observational signatures, thus paving the way for
testing, in the near future, one of the most peculiar predictions of general
relativity.
