  We have computed a grid of hydrostatic spherical COMARCS models for C stars
covering metallicities from [Z/H]=0 to -2 and values of the carbon excess [C-O]
from 6.41 to 9.15, plus some temperature sequences, where the amount of oxygen
and nitrogen is increased relative to a scaled solar element mixture. Such
abundance variations may appear during the late stages of stellar evolution.
Our study covers changes of [O/Z] and [N/Z] going up to +0.5. Based on the
atmospheric structures we have calculated synthetic spectra and photometry for
all of the models in a consistent way.
  The sequences with changed [O/Z] and [N/Z] can be used to derive correction
terms, which are applied to the colours predicted for a certain combination of
effective temperature, surface gravity, metallicity and carbon excess. If one
neglects these shifts in case of a variable oxygen amount, taking [C-O] instead
of C/O gives much better results, since the first quantity dominates the
formation of many important molecular species. For the warmer C giants with
weaker pulsation it is in principle possible to determine [C-O], [O/Z] or [N/Z]
from high resolution spectra, when the opacities in the radiative transfer
calculations for the models and observable properties are treated consistently.
The corresponding changes due to the abundances often become significantly
larger than the deviations caused by uncertainties of the stellar parameters or
by a optically thin dust shell. Photometric data and low or medium resolution
spectra are not sufficient to derive the mentioned quantities.
