Inflationary scenarios with scale-invariant spectral tensorial index

Next-to-leading order expressions related to Stewart-Lyth inverse problem are used to determine the inflationary models with a tensorial power spectrum described by a scale-invariant spectral index. Beyond power-law inflation, solutions are characterized by scale dependent scalar indices. These models can be used as assumption on the generation of primordial perturbations to test for scale dependence of the scalar index at large angular scales. If such a dependence is detected, a nonzero contribution of gravitational waves to the CMB spectrum must be expected.