Photoelectric Studies. V. Magnitudes and Colors of Classical Cepheid Variable Stars.

Magnitudes and colors, totaling 594 observations, have been determined for 32 classical cepheids. These variables can be divided into three types, A, B, and C, on the basis of the relations between the period, the color amplitude, and the light-amplitude. The cepheids of these three types are characterixed by the of Cephei, type A; n Aquilae, type B; and Geminorum, type C. A very close relation is also shown to exist between the amplitude of the radial-velocity variation and the amplitude of the light-variation. The existence of relations between the period and the color at maxium and minimum light, as well as a relation between period and luminosity of a cepheid, makes these variables powerful tools for sounding the interstellar medium. The use of these relations is demonstrated. The periodmagnitude relation, as obtained from the cepheids in the Magellanic Clouds, is discussed, and a method of determining the xero point of this relation from physical companions of galactic cepheids is demonstrated. The color-minimum4uminosity array of the classical cepheids of types A and B, obtained by combining the period-color and period-luminosity relations, is shown to be very similar to the colorluminosity array obtained from the brightest stars in globular clusters.