Most Algol-type variables are believed to be close, semi-detached binaries which have evolved from a rapid mass-transfer phase, which reversed the initial mass-ratio, to a relatively quiescent stage where only small-scale transient events perturb the dynamical and the photo-metrical properties of the system.

Of all Algol-systems, only the prototype Algol (β Per) itself is bright enough to be traceable in astronomical records reaching back more than 2000 years. Evidence based on medieval stellar lists and 19th-century visual observations strongly suggest that the intrinsic colour of Algol has been subject to substantial variations within historical times. On a few occasions Algol was apparently observed as reddish, which is at variance with its present blue-white appearance (B-V = −0.05 to +0.10).