If you look at the whole light curve, you can almost convince yourself that AC Herculis has a regular pulsation pattern, but when you start to look more closely, you'll see that the picture gets confusing. AC Her is an RV Tauri star, a highly evolved giant star that probably probably doesn't have much longer to live. These stars pulsate with periods of between 50 and 150 days, although you'll often see that they have something like alternating minima. Except they don't quite alternate! The RV Tauri stars are not regular variables with fixed pulsation periods even though they pulsate for the same reason as stars that are more regular. It's possible that they're so luminous and overextended in size that their pulsations tend to slide toward chaos more readily than more regular pulsators like Cepheids and even Miras do.