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A conservative constraint on the rest mass of the photon can be estimatedunder the assumption that the frequency dependence of dispersion fromastronomical sources is mainly contributed by the nonzero photon mass effect.Photon mass limits have been earlier set through the optical emissions of theCrab Nebula pulsar, but we prove that these limits can be significantlyimproved with the dispersion measure (DM) measurements of radio pulsars in theLarge and Small Magellanic Clouds. The combination of DM measurements ofpulsars and distances of the Magellanic Clouds provide a strict upper limit onthe photon mass as low as $m_{\gamma} \leq2.0\times10^{-45}~\rm{g}$, which isat least four orders of magnitude smaller than the constraint from the CrabNebula pulsar. Although our limit is not as tight as the current best result($\sim10^{-47}~\rm{g}$) from a fast radio burst (FRB 150418) at a cosmologicaldistance, the cosmological origin of FRB 150418 remains under debate; and ourlimit can reach the same high precision of FRB 150418 when it has anextragalactic origin ($\sim10^{-45}~\rm{g}$).

New Limits on the Photon Mass with Radio Pulsars in the Magellanic Clouds