Color Excesses for Supergiants and Classical Cepheids. I. Calibration of the Gband Photometry.

The determination of spectral types for cepheids on the MK system is critically examined. It is concluded that the anomalous strengthening of certain strong lines in cepheid spectra near maximum light results chiefly from a kinematical disturbance rather than from an excitation or ionization effect Spectral types for cepheids that are consistent with those of non-variable supergiants of the MK system are derived from a study of the G band. G-band strengths are measured using interference-filter techniques, and the measurements are correlated with intrinsic (B - V) color through observations of U Sgr and EV Sct, which are members of open clusters, and & Cep, which has a blue, presumably physical, companion. Supplementary spectral types are obtained for these and other cepheids from visual inspection of the G band on classification spectrograms. Intrinsic colors are given for class Ib stars from F0 to K0. Color excesses of high precision are derived for 19 cepheids; somewhat less precise excesses are given for 19 other cepheids and 7 non-variable supergiants. It is concluded that the Magellanic Cloud cepheids with periods between 2.5 and 45 days might have the same relation between (B - V) and log P as do the galactic cepheids if the Cloud cepheids are reddened, on the average, by about 0.15 mag. owing to interstellar material in the Clouds themselves.