The Spectral Sequence in Stars of Class S

Stars of spectral class S are the only ones which have not yet been subdivided according to the usual method of spectral classification. In a recent study of these stars, the writer was impressed by the great range in the values for the observable characteristics of the S stars. Furthermore, the S stars seem to be closely related to M stars, but in a peculiar manner; that is, on some criteria (apparent temperature, notably), they are most closely related to early M-type stars, while, on other criteria (mean period, density, and others), they resemble late M-type stars. Spectroscopically, there are conspicuous differences among the stars of Class S. At maximum light, the intensities of the ZrO bands, as well as of the TiO bands (if present), are largely different for different stars. As for the bright lines, Merrill1 has pointed out that the spectrum of R Cygni is remarkable because of the persistence of the iron lines throughout most of the light cycle. In S Ursae Majoris, on the other hand, the same lines are weak. The same contrast is shown in the intensities of λλ4511 and 4521. For these reasons, it seems important at this time to attempt a subdivision of stars of Class S. The variables, being the most extensively observed, have been considered first. Following the suggestion of Merrill,2 a subdivision has been attempted on the basis of the relative intensities of the TiO and ZrO bands. From an examination of plates for twenty-one variables, obtained and described by Merrill, it has been found that the spectra fall into five fairly distinct groups. These groups are indicated in Table I (p. 268). In explanation of the method of classification, it may be said that the effort has been made to base it on the appearance of the spectrum at maximum phase. It will be noted, however,