  We provide an analytical approach to the second-order Cosmic Microwave
Background (CMB) anisotropies generated by the non-linear dynamics taking place
at last scattering. We study the acoustic oscillations of the photon-baryon
fluid in the tight coupling limit and we extend at second-order the Meszaros
effect. We allow for a generic set of initial conditions due to primordial
non-Gaussianity and we compute all the additional contributions arising at
recombination. Our results are useful to provide the full second-order
radiation transfer function at all scales necessary for establishing the level
of non-Gaussianity in the CMB.
