RV Tau variables exhibit changes in luminosity which are tied to radial pulsations of their surfaces. Their changes in brightness are also correlated with changes in their spectral type. While at their brightest, the stars have spectral types F or G. At their dimmest, their spectral types change to K or M. The difference between maximum and minimum brightness can be as much as four magnitudes. The period of brightness fluctuations from one deep minimum to the next is typically around 30 to 150 days, and exhibits alternating primary and secondary minima, which can change relative to each other. For comparison with other type II Cepheids such as W Virginis variables, this formal period is twice the fundamental pulsation period. Therefore, although the approximate division between W Vir variables and RV Tau variables is at a fundamental pulsation period of 20 days, RV Tau variables are typically described with periods of 40–150 days.