At first the neutron star systems found to have QPOs were of a class (Z sources and atoll sources) not known to have pulsations. The spin periods of these neutron stars were unknown as a result. These neutron stars are thought to have relatively low magnetic fields so the gas does not fall mostly onto their magnetic poles, as in accreting pulsars. Because their magnetic fields are so low, the accretion disk can get very close to the neutron star before being disrupted by the magnetic field.