T Cas is a northern Mira with what appear to be traveling waves in the light curve. If you look at the maxima over the last several thousand days, you'll see that there appear to be multiple peaks that shift in time from cycle to cycle. The depth of the minima appear to change over time as well. Perhaps there is more than one pulsation mode at work in this star. Miras typically pulsate in the radial fundamental mode, meaning that while the center of the star is fixed in space, the entire interior of the star expands and contracts in the same direction with each pulsation. But there may be a shorter period overtone mode at work as well, and several other classes of variable star (like Cepheids and delta Scuti stars) can show the same behavior.