  The perturbations in the electron number density during recombination
contributes to the Cosmic Microwave Background bispectrum through second order
terms. Perturbations in the electron density can be a factor of ~5 larger than
the baryon density fluctuations on large scales as shown in the calculations by
Novosyadlyj. This raises the possibility that the contribution to bispectrum
arising from perturbations in the optical depth may be non-negligible. We
calculate this bispectrum and find it to peak for squeezed triangles and of
peak amplitude of the order of primordial non-Gaussianity of local type with
fNL of 0.05 ~ -1 depending on the l-modes being considered. This is because the
shape of the bispectrum is different from the primordial one although it peaks
for squeezed configurations, similar to the local type primordial
non-Gaussianity.
