  We study a new contribution to the polarization of the Cosmic Microwave
Background induced at the epoch of recombination by the second-order quadrupole
moment of the photon distribution. At second order in perturbation theory the
quadrupole moment is not suppressed by the inverse of the optical depth in the
tight coupling limit, as it happens at first order in perturbation theory. We
concentrate on the B-mode CMB polarization and find that such a novel
contribution constitutes a contamination in the detection of the primordial
tensor modes if the tensor to scalar ratio r is smaller than a few x 10^{-5}.
The magnitude of the effect is larger than the B-mode due to secondary
vector/tensor perturbations and the analogous effect generated during the
reionization epoch, while it is smaller than the contamination produced by the
conversion of polarization of type E into type B, by weak gravitational
lensing. However the lensing signal can be cleaned, making the secondary modes
discussed here the actual contamination limiting the detection of small
amplitude primordial gravitational waves if r is below \simeq 10^{-5}.
