  Changes of the 400 cm^{-1} peak in the c-axis conductivity of underdoped
YBa2Cu3O6.6 upon application of a parallel magnetic field reported by Kojima et
al. are shown to be consistent with the model where the peak is due to the
superfluid. Results of our calculations of the c-axis response of bilayer
compounds with well defined bilayer-split bands are presented and discussed.
For moderate values of the bilayer splitting (Delta E comparable to
2Delta_{max}) the spectra of the superconducting state exhibit an additional
mode which is due to the condensate and similar to the one of earlier
phenomenological approaches.
