Algol variables(Beta Persei stars) A subclass of eclipsing binary stars, named after Algol, where the brighter and more massive star is still on the main sequence while the less massive companion has evolved more and has become a subgiant. This seemingly contradicts the theory of stellar evolution, which predicts that more massive stars evolve more rapidly, and is known as the Algol paradox. It is explained (Crawford 1955) as a result of extensive mass transfer: the now less-massive star originally contained most of the system's mass, and it evolved rapidly beyond the main sequence. As it expanded, this star lost up to 85% of its mass to the companion (see W Serpentis star) to end up as a faint low-mass subgiant, while the companion became a massive hot and brilliant star, still on the main sequence. Mass transfer continues at a very slow rate in Algol systems, causing variations in the orbital period and feeble radio and X-ray emission.
As a result of the mass transfer, the two stars have the unusual property of being roughtly the same size (several times larger than the Sun) but having very different luminosities. They thus have a light curve characterized by deep primary minima when the dim subgiant eclipses the bright main-sequence star, alternating with scarcely detectable secondary minima when the subgiant is eclipsed.