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Light scalar field constraints from gravitational-wave observations of compact binaries

Scalar-tensor theories are among the simplest extensions of general relativity. In theories with light scalars, deviations from Einstein's theory of gravity are determined by the scalar mass m_s and by a Brans-Dicke-like coupling parameter ω_{BD}. We show that gravitational-wave observations of nonspinning neutron star-black hole binary inspirals can be used to set lower bounds on ω_{BD} and upper bounds on the combination m_s/\sqrt{ω_{BD}}$. We estimate via a Fisher matrix analysis that individual observations with signal-to-noise ratio ρwould yield (m_s/\sqrt{ω_{BD}})(ρ/10)<10^{-15}, 10^{-16} and 10^{-19} eV for Advanced LIGO, ET and eLISA, respectively. A statistical combination of multiple observations may further improve these bounds.