The primary star, called Algol A, is a spectral class B8 main sequence star of about 3 solar diameters, while the companion Algol B is a cooler spectral class K2 star with a diameter of about 3.5 solar diameters.
The center of mass for the system (denoted by an x) is just inside the primary star. It is believed that the shape of the secondary is distorted by the gravitational influence of the primary.
The partial eclipses (which cause light variation that is visible to the naked eye) are deepest when part of the hotter star is hidden and more shallow when part of the cooler star is hidden in eclipse; these eclipses occur with a period of 68.75 hours.