Possibility to Probe Gravitational Theory by Gravitational Wave

We study perturbations of relativistic stars in scalar-tensor theory of gravity and examine the effects of the scalar field on 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