  We present a study of the C K-edge using high-resolution LETGS Chandra
spectra of four novae during their super-soft-source (SSS) phase. We identified
absorption lines due to C~ii K$\alpha$, C~iii K$\alpha$ and C~ iii K$\beta$
resonances. We used these astronomical observations to perform a benchmarking
of the atomic data, which involves wavelength shifts of the resonances and
photoionization cross-sections. We used improved atomic data to estimate the
C~ii and C~iii column densities. The absence of physical shifts for the
absorption lines, the consistence of the column densities between multiple
observations and the high temperature required for the SSS nova atmosphere
modeling support our conclusion about an ISM origin of the respective
absorption lines. Assuming a collisional ionization equilibrium plasma the
maximum temperature derived from the ratio of C~ii/C~iii column densities of
the absorbers correspond to $T_{max}< 3.05\times10^{4}$ K.
