We have seen, in the last chapter, that stars varying their light in periods of less than fifty days stand apart in several important respects from those undergoing slower changes. The distinction is accentuated by the tendency apparent in each class to group its members as far as possible from the frontier-line of separation from the other. Thus, long periods for the most part exceed three hundred days, while a large majority of short periods fall below ten. Thirty-eight stars in all are reckoned as variable within fifty days; of these thirty-two complete an oscillation in less than twenty, twenty-seven (including two with imperfectly ascertained periods) in less than ten days. A comparison of figures 17 and 18 shows that, among short periods taken en masse, those of three to four days predominate; those of five to eight days when Algol variables are excluded.

Variables of short period are, as we have said, nearly all white or yellow stars. A very few are reddish; and one—W Virginis—is suspected to possess a banded spectrum. R Lyræ, a star of about 4·5 magnitude, with a superb spectrum of the third type, is nominally variable in forty-six days, but its changes are so trifling and so imperfectly rhythmical as to suggest that its proper place is with β Pegasi and α Herculis among stars affected by abortive periodicity.