Predicted Z_⊙ period-radius relations and Polaris’ interferometrically measured radius assuming the B18 parallax. Blue and red lines drawn are linear fits to the discretely computed models near the hot and cool IS edges; blue open squares mark computed models along the hot boundary of the first crossing. 1X denotes first crossing models, 3X third crossing models. P − R relations for FU Cepheids are shown for reference. The period and radius of a 7 M_⊙ 1X model (logP = 0.5904, log R∕R_⊙ = 1.7107) on the first crossing nearly coincides with the Polaris measurement (0.5987, 1.7114).