  A second--order perturbation theory approach is developed to calculate
temperature anisotropies in the cosmic microwave background. Results are given
for open universes and fluctuations corresponding to CDM models with either
Harrison-Zeldovich (HZ) or Lyth-Stewart-Ratra-Peebles (LSRP) primordial
energy--density fluctuation power spectrum. Our perturbation theory approach
provides a distinctive multipole contribution as compared to the primary one,
the amplitude of the effect being very dependent on normalization. For
low--$\Omega$ models, the contribution of the secondary multipoles to the
radiation power spectrum is negligible both for standard recombination and
reionized scenarios, with the 2--year COBE--DMR normalization. For a flat
universe this contribution is $\approx 0.1-10\%$ depending on the reionization
history of the universe and on the normalization of the power spectrum.
