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tokens: for mass loss may be necessary to obtain observed results, as done by Neilson et al. ( 2012a ). This is because the rate of period change is directly related to the mass-loss rate from taking the derivative of the period–mean density relation, typically . The greater the mass-loss rate the greater and more positive the rate of period change and if the average mass-loss rate is high enough it could cause more Cepheids to have positive rates of period change even while evolving on the second crossing of the instability strip where the rate of period change is expected to be less than zero. 4. Results For each initial rotation rate and convective core overshooting value we compute the relative fraction of models with positive and negative rates of period change. We find that regardless of the initial rotation rate or convective core overshooting parameter, the fraction of Cepheids with positive period change is about 90% and the fraction with negative period change is about 10%, as can be seen in Table 1 . This suggests that period change, at least statistically, is insensitive to the physics of core overshooting and rotational mixing. Table 1 Predicted Fractions of Cepheids with Positive and Negative Period Changes This insensitivity has been noted before for overshooting. Neilson et al. ( 2012b ) and Neilson ( 2014 ) found similar results in that overshooting acts to change the mass–luminosity relation for Cepheids. That is, in terms of period change, Cepheids with different masses have the same rates of period change for different values of core overshooting. This is because each Cepheid crossing of the instability strip depends on the physics driving stellar evolution. During the first crossing the evolutionary timescale is related to the Kelvin–Helmholtz timescale, hence it is proportional to the mass of the star. Evolution along the second crossing of the instability strip occurs as the star generates energy through mostly hydrogen shell-burning and is beginning helium core-burning. The timescale for blueward evolution is driven by the timescale for the star to have shell-burning, but not have ignited significant helium core fusion. The third crossing is determined by helium core-burning, hence its timescale is also a function of helium core mass. Over the mass range we consider in this paper, overshooting will not change the timescales, and thus the fraction of Cepheids on the first crossing, while the impact on the second and third crossings also remains unchanged. The same argument is also true for rotation. Rotational mixing impacts the mass of the helium core and rotation can change the shape of the Cepheid blue loop. However, neither appears to impact the relative timescales of evolution significantly. The fraction of Cepheids with positive period change decreases to about 77% for models with zero convective core overshooting. The fraction is smaller, but not enough to be consistent with the observed fraction of 67%. This