Pulsars in close binary systems with white dwarfs or other neutron stars make ideal laboratories for testing the predictions of gravitational radiation and self-gravitational effects. We report new timing measurements of the pulsar–white-dwarf binary PSR J1141–6545. The orbit is found to be decaying at a rate of $1.04\pm 0.06$ times the general relativistic prediction and the Shapiro delay is consistent with the orbital inclination angle derived from scintillation measurements. The system provides a unique testbed for tensor-scalar theories of gravity. Our measurements place stringent constraints in the theory space, with a limit of ${\alpha }_0^2<2.1\times {10}^{-5}$ for weakly nonlinear coupling and an asymptotic limit of ${\alpha }_0^2<3.4\times {10}^{-6}$ for strongly nonlinear coupling (where ${\alpha }_0$ is the linear coupling strength of matter to an underlying scalar field), which is nearly 3 times smaller than the Cassini bound (${\alpha }_0^2\approx {10}^{-5}$).

• Received 21 January 2008

DOI:https://doi.org/10.1103/PhysRevD.77.124017