  We derive the perturbation equations for relativistic stars in scalar-tensor
theories of gravity and study the corresponding oscillation spectrum. We show
that the frequency of the emitted gravitational waves is shifted proportionally
to the scalar field strength. Scalar waves which might be produced from such
oscillations can be a unique probe for the theory, but their detectability is
questionable if the radiated energy is small. However we show that there is no
need for a direct observation of scalar waves: the shift in the gravitational
wave spectrum could unambiguously signal the presence of a scalar field.
