  A large class of modified theories of gravity used as models for dark energy
predict a propagation speed for gravitational waves which can differ from the
speed of light. This difference of propagations speeds for photons and
gravitons has an impact in the emission of gravitational waves by binary
systems. Thus, we revisit the usual quadrupolar emission of binary system for
an arbitrary propagation speed of gravitational waves and obtain the
corresponding period decay formula. We then use timing data from the
Hulse-Taylor binary pulsar and obtain that the speed of gravitational waves can
only differ from the speed of light at the percentage level. This bound places
tight constraints on dark energy models featuring an anomalous propagations
speed for the gravitational waves.
