The luminosity of RV Tau variables is typically a few thousand times the sun, which places them at the upper end of the W Virginis instability strip. Therefore, RV Tau variables along with W Vir variables are sometimes considered a subclass of Type II Cepheids. They exhibit relationships between their periods, masses, and luminosity, although not with the precision of more conventional Cepheid variables. Although the spectra appear as supergiants, usually Ib, occasionally Ia, the actual luminosities are only a few thousand times the sun. The supergiant luminosity classes are due to very low surface gravities on pulsating low-mass and rarefied stars.