Magellanic Cloud Cepheids: An extrinsic model of the anomalies

The Cepheids in the Small Magellanic Cloud are bluer on the average than their counterparts in the Galaxy. Previous attempts have been only partially successful in explaining the observed differences. We present a model in which assume that the SMC Cepheids are intrinsically the same as those in the Galaxy. First we show that the principal effects of an early-type companion on the Cepheid light and color curves are to make the Cepheid appear bluer at minimum light and to produce a loop in the color-diagram of the Cepheid. The results indicate a simple way to determine the spectral type of the Cepheid from broad-band photometry. Next we calculate the probability that the observations of the SMC Cepheids are contaminated by the presence of an early-type star in the field of view. We find that such contamination can occur in the central regions but is less likely far from the center of the SMC. We examine the effect that contamination has on the period-luminosity and ridge-line relations and show that the results agree with the observations if between 20% and 40% of the SMC Cepheids have companions with a spectral type of B2 V to B8 V. This percentage is consistent with, but somewhat higher than, the occurrence of binaries in the Galaxy. The extra companions in the SMC may be due to chance coincidences in position