Electromagnetic radiation from binary stars mediated by ultralight scalar

We present the electromagnetic (EM) dipole radiation flux from an eccentric Keplerian binary endowed with scalar charges, in the presence of scalar-photon coupling [Formula: see text] or [Formula: see text]. The scalar radiation is suppressed for orbital frequency below the scalar mass, while the scalar-mediated indirect EM radiation survives. We examine the constraints imposed on the scalar-photon and scalar-charge couplings by the current observational data of pulsar binaries, in case that the scalar charge is given by the muon number. The general extensions of the calculation to the quadrupole order and hyperbolic orbit are also discussed.