Eclipsing variables are not true (intrinsic) variables but rather are binary star systems, i.e., pairs of stars revolving around a common center of mass. The apparent brightness of an eclipsing variable fluctuates because the orbit of the pair is seen edgewise, so that first one star and then the other regularly blocks the light of its companion. Best known of this type is Algol (Beta Persei).
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.