  The recent observations of gravitational-wave and electromagnetic emission
produced by the merger of the binary neutron-star system GW170817 have opened
the possibility of using standard sirens to constrain the value of the Hubble
constant. While the reported bound of $H_0=70_{-8}^{+12}$ at $68 \%$ C.L. is
significantly weaker than those recently derived by observations of Cepheid
variables, it does not require any form of cosmic distance ladder and can be
considered as complementary and, in principle, more conservative. Here we
combine, for the first time, the new measurement with the Planck Cosmic
Microwave Background observations in a $12$ parameters extended $\Lambda$CDM
scenario, where the Hubble constant is weakly constrained from CMB data alone
and bound to a low value $H_0=55^{+7}_{-20}$ km/s/Mpc at $68 \%$ C.L. We point
out that the non-Gaussian shape of the GW170817 bound makes lower values of the
Hubble constant in worst agreement with observations than what expected from a
Gaussian form. The inclusion of the new GW170817 Hubble constant measurement
therefore significantly reduces the allowed parameter space, improving the
cosmological bounds on several parameters as the neutrino mass, curvature and
the dark energy equation of state.
