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As explained in detail above, the evolution of a NS, by its nature, must be studied jointly with the evolution of normal stars. This problem was discussed qualitatively by Bisnovatyi-Kogan and Komberg (1974), van den Heuvel (1977) and Lipunov (1982a).
In Figure 10 we show possible evolutionary tracks of NS. As a rule, the NS is generated at the instant when the companion lies on the main sequence (track b). During the first - years, the star is in the ejector state usually unobservable due to absorption in the stellar wind from the normal star. The period of the NS increases in accordance with the magnetic dipole losses. After that, plasma penetrates inside the light cylinder and the NS passes first to the propeller stage and then to the accretor stage. By this time, the normal star leaves the main sequence and the stellar wind is intensified. This results in the formation of a bright X-ray pulsar. The period of the NS is stabilized close to its equilibrium value. Finally, the normal star fills the Roche lobe and the accretion rate suddenly increases; the NS moves first to the right and then vertically down on the diagram. In other words, the NS moves into the supercritical stage SA (superaccretor). Its period tends to a new equilibrium value (see Lipunov, 1992):
After the mass exchange, only the helium core of the normal star is left (WR star), a separate system is formed (in some systems the NS has spiraled completely into the center of its companion, and a Thorne-Zytkow object is formed), and the NS again becomes a propeller or an ejector. Accretion is hampered by the rapid rotation. This is probably a reason for the observed lack of X-ray pulsars in pairs with Wolf-Rayet stars (Lipunov, 1982e). The helium star evolves on a short time-scale ( yr), so the companion NS does not have time to spindown considerably: after the explosion of the normal star, the system is disrupted and the NS becomes an ejector again, that is, it may be observed as a radiopulsar.
The ``loop-shaped'' track discussed in Section 4.1. shows another evolutionary behavior of a NS formed in the process of mass exchange in the binary system:
The total lifetime of a NS in a binary system depends on the lifetime of the normal star and the parameters of the binary system. However, the rate of transition from one stage to another is proportional to the NS magnetic field.