Gravitational-radiation losses from the pulsar-white-dwarf binary PSR J1141-6545

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 +/- 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(2)(0) < 2.1 x 10(-5) for weakly nonlinear coupling and an asymptotic limit of alpha(2)(0) < 3.4 x 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(2)(0) approximate to 10(-5)).