On the duration of the subsonic propeller state of neutron stars in wind-fed mass-exchange close binary systems

The condition for the subsonic propeller- accretor state transition of neut ron stars in wind-fed mass-exchange binary systems is discussed. I show tha t the value of the break period. at which the neutron star change its state to accretor, presented by Davies & Pringle (1981) is underestimated by a f actor of 7.5. The correct value P-br similar or equal to 450 mu (16/21)(30) (M) over dot(15)(-5/6) (M/M-circle dot)(-4/21)s. This result forced us to reconsider some basic conclusions on the efficiency of the propeller spindo wn mechanism.