Geoff Fox Close binaries are stars that are physically so close to each other that they actually influence each other's evolution.
Astronomers know that the more massive a star is the more rapidly it evolves.
However, there is one class of objects for which this does not seem to be the case. These stars are called Algol systems where the more massive star is less evolved than its companion. A third type of close binary is the massive x-ray binary. This binary system consists of an O and B type supergiant and a neutron star or black hole that acts as the x-ray source. We believe that matter from the supergiant may be accreting on the neutron star.