  We report on results of our analysis of published experimental data of the
in-plane infrared response of two representative underdoped
high-$T_{\mathrm{c}}$ cuprate superconductors, focusing on a characteristic gap
feature in the spectra of the real part of the conductivity and the
corresponding structures of the memory function, that develop below a
temperature $T^{\mathrm{ons}}$ considerably higher than $T_{\mathrm{c}}$.
Several arguments based on comparisons of the data with results of our
calculations are provided indicating that the features are due to
superconductivity and that $T^{\mathrm{ons}}$ marks the onset of a precursor
superconducting phase.
