RV Tauri variable

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.

The brightest member of the class, R Scuti, is an RVa type, with an apparent magnitude varying from 4.6 to 8.9 and a formal period of 146.5 days.

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.

The supergiant luminosity classes are due to very low surface gravities on pulsating low-mass and rarefied stars.

Late AGB stars become increasingly unstable, show large amplitude variations as Mira variables, experience thermal pulses as internal hydrogen and helium shells alternate fusing, and rapidly lose mass.

Although a post-AGB crossing of the instability strip should happen in a period measured in thousands of years, even hundreds for the more massive examples, the known RV Tau stars have not shown the secular increase in temperature that would be expected.

The main sequence progenitor of this type of star has a mass near to that of the sun, although they have already lost about half of that during red giant and AGB phases.