Inflation coupled to a Gauss-Bonnet term
  The newly released Planck CMB data place tight constraints on slow-roll
inflationary models. Some of commonly discussed inflationary potentials are
disfavored due mainly to the large tensor-to-scalar ratio. In this paper we
show that these potentials may be in good agreement with the Planck data when
the inflaton has a non-minimal coupling to the Gauss-Bonnet term. Moreover,
such a coupling violates the consistency relation between the tensor spectral
index and tensor-to-scalar ratio. If the tensor spectral index is allowed to
vary freely, the Planck constraints on the tensor-to-scalar ratio are slightly
improved.
