Period Changes of the SMC Cepheids from the Harvard, OGLE and ASAS Data

Abstract

Comparison of the old observations of Cepheids in the Small Magellanic Cloud from the Harvard data archive, with the recent OGLE and ASAS observations allows an estimate of their period changes. All of matched 557 Cepheids are still pulsating in the same mode. One of the Harvard Cepheid, HV 11289, has been tentatively matched to a star which is now apparently constant. Cepheids with log P > 0.8 show significant period changes, positive as well as negative. We found that for many stars these changes are significantly smaller than predicted by recent model calculations. Unfortunately, there are no models available for Cepheids with periods longer than approximatelly 80 days, while there are observed Cepheids with periods up to 210 days.