A principal goal of gravitational-wave astronomy is to constrain the neutron star equation of state (EOS) by measuring the tidal deformability of neutron stars. The tidally induced departure of the waveform from that of a point particle [or a spinless binary black hole (BBH)] increases with the stiffness of the EOS. We show that causality (the requirement that the speed of sound be less than the speed of light for a perfect fluid satisfying a one-parameter equation of state) places an upper bound on tidal deformability as a function of mass. Like the upper mass limit, the limit on deformability is obtained by using an EOS with $v_{\text{sound}}=c$ for high densities and matching to a low density (candidate) EOS at a matching density of order nuclear saturation density. We use these results and those of Lackey et al. [Phys. Rev. D 89, 043009 (2014)] to estimate the resulting upper limit on the gravitational-wave phase shift of a black hole–neutron star (BHNS) binary relative to a BBH. Even for assumptions weak enough to allow a maximum mass of $4M_{\odot }$ (a match at nuclear saturation density to an unusually stiff low-density candidate EOS), the upper limit on dimensionless tidal deformability is stringent. It leads to a still more stringent estimated upper limit on the maximum tidally induced phase shift prior to merger. We comment in an appendix on the relation among causality, the condition $v_{\text{sound}}<c$, and the condition $dp/d\epsilon <1$ for the effective EOS governing the equilibrium star.

• Received 31 January 2017

DOI:https://doi.org/10.1103/PhysRevD.95.083014