BEHAVIOUR OF CEPHEID VARIABLES IN COLOUR-COLOUR DIAGRAM

We used the catalogue of the mean light-curves and colours of the Cepheids in the UBV system (Nikolov, 1968), for 240 Cepheids, and constructed the curves which represent plots of their U-B versus B-V colour indices. These curves form three groups: linear curves, open curves, and curves possessing a loop. An opinion that the presence of a loop may be a criterion for Population II objects is not confirmed. It is shown that the slope of such curves (corrected for reddening) with respect to the B-V axis increases with the period. The area delimited by the curves on the U-B/B-V diagram remains almost the same up to a period of about 9 to 10 days, whereafter it increases with the period. The possible causes for this behaviour of the Cepheids on the colour-colour diagram, which is connected with the ultraviolet emission of these stars, are discussed. © 1969 D. Reidel Publishing Company.