Classification of the S-Type Stars.

In this survey, spectra of type S are defined as those having bands of ZrO strong enough to he detected with low dispersion. This limitation is made because the group will be a useful one only if most or all of the members can be recognized on slitless spectrograms and if they are enough alike to give significance to their statistical properties. Stars with weaker ZrO bands are termed "MS" stars, and 14 of these are tabulated. Seven other stars showing neither ZrO nor TiO bands in their spectra are set aside as related more distantly to the S-type stars. The 69 S-type stars of the main catalogue are classified from the relative intensities of their ZrO and TiO bands into a two-dimensional system, in which the variables appear to be temperature and chemical composition. The lines of evidence bearing on the luminosities of the S stars-their motions and galactic distribution, as well as the classification by Feast of the faint companion of it' Gruis-suggest that these stars belong among the giants and bright giants (luminosity classes III and II), possibly spanning a considerable range in absolute magnitude. Preliminary counts of M, MS, S, and C stars in the giant branch of the cooler stars with banded spectra show that roughly 10 per cent of them show appreciable strengthening of the S characteristics-the lines of the heavier metals and the bands of their oxides.