  The equation of state (EOS) for partially ionized carbon, oxygen, and
carbon-oxygen mixtures at temperatures 3\times10^5 K <~ T <~ 3\times10^6 K is
calculated over a wide range of densities, using the method of free energy
minimization in the framework of the chemical picture of plasmas. The free
energy model is an improved extension of our model previously developed for
pure carbon (Phys. Rev. E, 72, 046402; arXiv:physics/0510006). The internal
partition functions of bound species are calculated by a self-consistent
treatment of each ionization stage in the plasma environment taking into
account pressure ionization. The long-range Coulomb interactions between ions
and screening of the ions by free electrons are included using our previously
published analytical model, recently improved, in particular for the case of
mixtures. We also propose a simple but accurate method of calculation of the
EOS of partially ionized binary mixtures based on detailed ionization balance
calculations for pure substances.
