Close binaries and their significance in the theory of evolution of stars

The general principles of investigations of close binaries by photometric and spectral methods, and theoretically - on the basis of concepts of celestial mechanics, are outlined. It is shown that when a star of the binary system has filled out the limiting Roche figure in the course of its evolution, the evolution of the star proceeds at a faster rate and leads to the formation of such high-density stars as white dwarfs and even collapsing stars (which otherwise could hardly be observed). In binary systems, such stars can be detected even from gravitational effects or from the transformation of the stellar plasma in the passage from one to the other component. The possibility of observing black holes is examined.