Direct detection of gravitational waves can measure the time variation
  of the Planck mass
  The recent discovery of a $\gamma$-ray counterpart to a gravitational wave
event has put extremely stringent constraints on the speed of gravitational
waves at the present epoch. In turn, these constraints place strong theoretical
pressure on potential modifications of gravity, essentially allowing only a
conformally-coupled scalar to be active in the present Universe. In this paper,
we show that direct detection of gravitational waves from optically identified
sources can also measure or constrain the strength of the conformal coupling in
scalar--tensor models through the time variation of the Planck mass. As a first
rough estimate, we find that the LISA satellite can measure the dimensionless
time variation of the Planck mass (the so-called parameter $\alpha_M$) at
redshift around 1.5 with an error of about 0.03 to 0.13, depending on the
assumptions concerning future observations. Stronger constraints can be
achieved once reliable distance indicators at $z>2$ are developed, or with GW
detectors that extend the capabilities of LISA, like the proposed Big Bang
Observer. We emphasize that, just like the constraints on the gravitational
speed, the bound on $\alpha_M$ is independent of the cosmological model.
