SIMBAD references

In this paper we first summarise earlier results about the randomness of pulse period variations in wind fed X-ray pulsars. Then we present two types of models for the random spin-up and spin-down of rotating neutron stars which accrete mass from a stellar wind. We show that the irregularities in accretion flows which were discovered in numerical models have too low amplitudes in order to explain the observations. Therefore we discuss an alternative model which is based upon the existence of a Kepler disc around the neutron star. We argue that such a model can give an explanation of the observations.