Volume 48 number 1 (2020)
(Abstract only) Type II Cepheids are believed to be evolved, low-mass stars. Observed rates of period change for these variables provide important tests for theoretical models of their evolution. If we consider those short-period type II Cepheids (sometimes called BL Her stars) which are metal-poor, we find that observed rates of period change are broadly consistent with predictions that they are stars evolving from the blue horizontal-branch toward the asymptotic red giant branch. There are, however, irregular variations in the rates of period change that are not predicted theoretically, and some of the details of the evolutionary models do not always match observations. Longer-period type II Cepheids (called W Virginis stars) have sometimes been understood to be stars undergoing loops to the blue from the asymptotic red giant branch caused by thermal pulse instabilities. However, not all theoretical models predict such loops. Some W Virginis variables exhibit period changes consistent with those from the loop models, but others show more irregular period fluctuations.