  This talk is based on my work in collaboration with Thibault Damour since
1991. Unified theories, like superstrings, predict the existence of scalar
partners to the graviton. Such theories of gravity can be very close to general
relativity in weak-field conditions (solar-system experiments), but can deviate
significantly from it in the strong-field regime (near compact bodies, like
neutron stars). Binary pulsars are thus the best tools available for testing
these theories. This talk presents the four main binary-pulsar experiments, and
discusses the constraints they impose on a generic class of tensor-scalar
theories. It is shown notably that they rule out some models which are strictly
indistinguishable from general relativity in the solar system. This illustrates
the qualitative difference between binary-pulsar and solar-system tests of
relativistic gravity.
