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.
