Predicted rates of period change for Z_⊙ FO Cepheids (lines) and observed values for Galactic Cepheids (Turner et al. 2006, markers include both FO and FU Cepheids). Adapted from Fig. 13 in A16 to illustrate predictions for FO Cepheids. First IS crossings are labeled “1X”, third IS crossings “3X”. Polaris (green filled circle) and threeother Cepheids (DX Gem, BY Cas, and HD 344787; yellow markers) exhibit much faster Ṗ than the bulk of Cepheids and are likely crossing the IS for the first time. Line styles distinguish initial rotation rates of the models: dotted red = no rotation, solid black = average rotation (ω = 0.5), dashed blue = fast rotation (ω = 0.9). Groups of lines correspond to different initial mass or IS crossing. Each line segment represents an IS crossing (left is hot IS boundary).