The effective temperature and metallicity of CM Draconis

We compare observations of the binary system CM Draconis with synthetic spectra computed using the stellar atmosphere code phoenix. Spectroscopic observations from 0.40 to 2.41 μm, combined with photometry and the accurately known surface gravity, enable us to estimate the temperature and metallicity using detailed spectral synthesis. We find discrepancies between the analysis of the infrared and optical spectra: while the optical spectral energy distribution (SED) yields a metal-rich olution with Teff= 3000 K, the infrared SED yields around 3200 K with — 0.8 ≤ [M/H] ≤ — 0.6, compatible with the high space motion of the system. The low-metallicity characteristics of the infrared SED could be real, and are partly supported by a detailed analysis of the atomic lines in the optical region. Although the known incompleteness of the TiO and H2O line lists in the models used, as well as problems with the observational data, will cause systematic errors, we suggest that CM Draconis could be a chemically peculiar system in that it cannot be modelled with solar abundances.