Period Changes in Population II Cepheids: TX Del and W Vir

Abstract

We have used the Hipparcos database of epoch photometry to study the variability of two Population II Cepheids: TX Del and W Vir. We have constructed (O-c) diagrams for these stars, using the new data and data from the literature. The period of TX Del is decreasing, at a rate which is consistent with evolutionary predictions. THe periodchange of W Vir is uncertain because of the many recent gaps in the data of this star. We urge AAVSO visual and photoelectric observers to monitor Population II Cepheids more systematically, since amny of these stars have unstable periods, and at least one has an unstable amplitude.