  We address the question of how one can modify the inflationary tensor
spectrum without changing at all the successful predictions on the curvature
perturbation. We show that this is indeed possible, and determine the two
quadratic curvature corrections that are free from instabilities and affect
only the tensor sector at the level of linear cosmological perturbations. Both
of the two corrections can reduce the tensor amplitude, though one of them
generates large non-Gaussianity of the curvature perturbation. It turns out
that the other one corresponds to so-called Lorentz-violating Weyl gravity. In
this latter case one can obtain as small as 65% of the standard tensor
amplitude. Utilizing this effect we demonstrate that even power-law inflation
can be within the 2$\sigma$ contour of the Planck results.
