Reddenings of FGK supergiants and classical Cepheids from spectroscopic data

Accurate and homogeneous atmospheric parameters (T_eff,logg,V_t, [Fe/H]) are derived for 74 FGK non-variable supergiants from high-resolution, high signal-to-noise ratio, echelle spectra. Extremely high precision for the inferred effective temperatures (10-40K) is achieved by using the line-depth ratio method. The new data are combined with atmospheric values for 164 classical Cepheids, observed at 675 different pulsation phases, taken from our previously published studies. The derived values are correlated with unreddened B - V colours compiled from the literature for the investigated stars in order to obtain an empirical relationship of the form (B - V)₀ = 57.984 - 10.3587(logT_eff)² + 1.67572(logT_eff)³ - 3.356logg + 0.0321V_t + 0.2615[Fe/H] + 0.8833(logg)(logT_eff). The expression is used to estimate colour excesses E(B - V) for individual supergiants and classical Cepheids, with a precision of +/-0.05 mag for supergiants and Cepheids with n = 1-2 spectra, reaching +/-0.025mag for Cepheids with n > 2 spectra, matching uncertainties for the most sophisticated photometric techniques. The reddening scale is also a close match to the system of space reddenings for Cepheids. The application range is for spectral types F0-K0 and luminosity classes I and II.