  (abridged) We report the results of a 10-year timing campaign on PSR
J1738+0333, a 5.85-ms pulsar in a low-eccentricity 8.5-hour orbit with a
low-mass white dwarf companion (...) The measurements of proper motion and
parallax allow for a precise subtraction of the kinematic contribution to the
observed orbital decay; this results in a significant measurement of the
intrinsic orbital decay: (-25.9 +/- 3.2) \times 10^{-15} s/s. This is
consistent with the orbital decay from the emission of gravitational waves
predicted by general relativity, (-27.7 +1.5/-1.9) \times 10^{-15} s/s (...).
This agreement introduces a tight upper limit on dipolar gravitational wave
emission, a prediction of most alternative theories of gravity for asymmetric
binary systems such as this. We use this limit to derive the most stringent
constraints ever on a wide class of gravity theories, where gravity involves a
scalar field contribution. When considering general scalar-tensor theories of
gravity, our new bounds are more stringent than the best current solar-system
limits over most of the parameter space, and constrain the matter-scalar
coupling constant {\alpha}_0^2 to be below the 10^{-5} level. For the special
case of the Jordan-Fierz-Brans-Dicke, we obtain the one-sigma bound
{\alpha}_0^2 < 2 \times 10^{-5}, which is within a factor two of the Cassini
limit. We also use our limit on dipolar gravitational wave emission to
constrain a wide class of theories of gravity which are based on a
generalization of Bekenstein's Tensor-Vector-Scalar gravity (TeVeS), a
relativistic formulation of Modified Newtonian Dynamics (MOND).
