Constraints on general second-order scalar-tensor models from   gravitational Cherenkov radiation
  We demonstrate that the general second-order scalar-tensor theories, which have attracted attention as possible modified gravity models to explain the late time cosmic acceleration, could be strongly constrained from the argument of the gravitational Cherenkov radiation. To this end, we consider the purely kinetic coupled gravity and the extended galileon model. In these models, the propagation speed of tensor mode could be less than the speed of light, which puts very strong constraints from the gravitational Cherenkov radiation. 
Rampei Kimura, Kazuhiro Yamamoto
astro-ph.CO gr-qc hep-th