Algol variables or Algol-type binaries are a class of eclipsing binary stars that are related to the prototype member of this class, β Persei (Beta Persei, Algol) from an evolutionary point of view. An Algol binary is a semidetached binary system where the primary component is an early type, main sequence star that does not fill its Roche lobe, while the cooler, fainter, larger, less massive secondary component lies above the main sequence in a Hertzsprung–Russell diagram and fills the Roche lobe. Early in its history, the secondary star would have been more massive, evolving first to overfill its Roche lobe. After rapid mass exchange, the lobe-filling star became less massive than its companion.