  A stochastic background of relic gravitational waves is achieved by the so
called adiabatically-amplified zero-point fluctuations process derived from
early inflation. In principle, it provides a distinctive spectrum of relic
gravitational waves. In the framework of scalar-tensor gravity, we discuss the
scalar modes of gravitational waves and the primordial production of this
scalar component which is generated beside tensorial one. We discuss also the
upper limit for such a relic scalar component with respect to the WMAP
constraints.
