Binary stars and their role in modern astronomy

The results of investigations of binary stars are discussed in a popular form. The problem of interaction in tight binary systems is considered. The interaction of the stars in these systems leads to changes of evolutional character. One of the phenomena related to binary stars is called apsidal motion in the eclipse system. Experimental data testify to the extremely slo apsidal motion in the DI Hercules system which contradicts to the calculations within the framewrk of General Relativity Theore