Thanks to the success of X-ray astronomy, the problem of cramped binary stars has become one of the central in astrophysics. The monograph sets out modern methods and results of research on close binary stars and information about their fundamental characteristics - masses, radii and temperatures. This makes tight binary stars a powerful tool for investigating the physics and evolution of stars, as well as for discovering and studying fundamentally new objects in the universe - neutron stars and black holes. The monograph can be useful to undergraduate and graduate students, professors and university professors, as well as scientists interested in the problems of star physics and relativistic astrophysics.