On the intrinsic colours of cepheids

Some consequences from new data on the photometrically obtained intrinsic colours are considered. It is shown that the small amplitude and almost sinusoidal light-curve cepheids (Cs-subtype) increase their pulsation amplitude towards the red instability strip edge, differing from the other galactic cepheids. This feature is discussed in connection with Efremov's (1968) hypothesis that the Cs-cepheids first cross the instability strip from left to right. The galactic cepheid period-colour relation obtained by Dean, Warren and Cousins (1978) satisfies rather well the LMC cepheid observations and, consequently, considerations for the period-luminosity relations are made. The residuals from the PL relation proposed by us (AA line instead of Gascoigne's BB line in Figure 3) correlate with the colour residuals from the DWC period-colour relation V/(B–V) being equal to 2.7 (Figure 4). The luminosity effect as a possible cause of the descrepancy between the spectroscopic cepheid colours and the photometric colours is briefly discussed.