An eclipsing binary star is a binary star with the orbital plane of the two stars angled so it is nearly in the line of sight of the observer. The pair of stars undergo mutual eclipses. Algol (β Persei) is the best-known example of an eclipsing binary. Algol is actually a triple-star system, in which the large and bright primary Algol A is regularly eclipsed by the dimmer Algol B every 2.87 days as seen from Earth.
The animation was created using near-infrared images from the CHARA interferometer. The numbers in the corner represent the relative phase of the orbit. The stars are so close together (about 1/16 of the distance between the Earth and the Sun) that Algol A is slowly stripping matter out of Algol B.
Image Credit: Fabien Baron (CC BY-SA 3.0)