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Period-color (PC) relations may be used to study the interaction of thestellar photosphere and the hydrogen ionization front (HIF). RR Lyraes (RRLs)and long period classical Cepheids (P > 10d) have been found to exhibitdifferent PC behavior at minimum and maximum light which can be explained bythe HIF-photosphere interaction based on their location on the HR diagram. Inthis work, we extend the study to include type II Cepheids (T2Cs) with an aimto test the HIF-photosphere interaction theory across a broad spectrum ofvariable star types. We find W Vir stars and BL Her stars to have similar PCrelations as those from long period and short period classical Cepheids,respectively. We also use MESA to compute RRL, BL Her and classical Cepheidmodels to study the theoretical HIF-photosphere distance and find the resultsto be fairly consistent with the HIF-photosphere interaction theory.

A study of the stellar photosphere-hydrogen ionisation front interaction in pulsating variables using period-color relations