B-mode polarization of the CMB from the second-order photon quadrupole

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}.