 For example, the dividing line between BL Her and W Vir types is quoted at anything between 4 and 10 days, with no obvious division between the two.RV Tau variables may not have obvious alternating minima, while some W Vir stars do.Nevertheless, each type is thought to represent a distinct different evolutionary stage, with BL Her stars being helium core burning objects moving from the horizontal branch towards the asymptotic giant branch (AGB), W Vir stars undergoing hydrogen or helium shell burning on a blue loop, and RV Tau stars being post-AGB objects at or near the end of nuclear fusion.