One of the few experimental handles on the nonlinear properties of the gravitational interaction is to test the ‘‘strong equivalence principle,’’ i.e., to test whether the ratio ${\mathit{m}}_{\mathrm{gravitational}}$/${\mathit{m}}_{\mathrm{inertial}}$ is 1 for self-gravitating bodies. We point out that existing observational data on the class of small-eccentricity long-orbital-period binary pulsars already provide a limit (namely ‖${\mathit{m}}_{\mathit{g}}$/${\mathit{m}}_{\mathit{i}}$-1‖<1.1×${10}^{\mathrm{-}2}$; 90% C.L.) which goes beyond corresponding solar-system limits in probing strong-gravitational-field effects. Possible observational ways of improving this limit are suggested.

• Received 20 February 1991

DOI:https://doi.org/10.1103/PhysRevLett.66.2549