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In the very first years after the discovery of radiopulsars, it became clear that for some reason they avoid forming binary systems. Three hypotheses were put forward to explain such a strange incompatibility.
Suppose that a normal star loses matter in a spherically symmetric way. In this case, the optical thickness associated with the free-free absorption in the stellar wind is
where K is the temperature of the stellar wind, cm is the wavelength of the radiofrequency radiation, and a is the semi-major axis of the orbit expressed in units of solar radii. In massive binary systems with , we have . Hence the stellar wind is opaque even for very wide systems ( ).
Which of the above three mechanisms is responsible for the deficit of radiopulsars in binary systems with normal stars? To answer this question correctly, a more detailed analysis of the evolution of normal and neutron stars in binary system is needed. Recent studies (see Lipunov et al., 1995d and Section 7 for more detail) suggest that the ill-defined conditions of stellar wind transparency turn out to be of a rather small significance since after the first supernova explosion the distribution of binaries by optical thickness of the wind is fairly flat on a logarithmic scale (which reflects the initial flat binary distribution on semimajor axes; Abt, 1983).