  We study the scintillation produced by time-varying gravitational fields
within scalar-tensor theories of gravity. The problem is treated in the
geometrical optics approximation for a very distant light source emitting quasi
plane monochromatic electromagnetic waves. We obtain a general formula giving
the time dependence of the photon flux received by a freely falling observer.
In the weak-field approximation, we show that the contribution to the
scintillation effect due to the focusing of the light beam by a gravitational
wave is of first order in the amplitude of the scalar perturbation. Thus
scalar-tensor theories contrast with general relativity, which predicts that
the only first-order effect is due to the spectral shift. Moreover, we find
that the scintillation effects caused by the scalar field have a local
character: they depend only on the value of the perturbation at the observer.
This effect provides in principle a mean to detect the presence of a long range
scalar field in the Universe, but its smallness constitutes a tremendous
challenge for detection.
