  A new class of high-mass X-ray binaries (HMXBs) --- supergiant fast X-ray
transients (SFXTs) --- are discovered by INTEGRAL, which are associated with OB
supergiants and present long spin periods. Observational evidence indicates
that some accreting neutron stars in HMXBs display accretion reversals. It has
been suggested that the inverted torque can lead to a very slow rotator.
According to three characteristic radii in wind-fed accretion, we developed a
retrograde accretion scenario and divided the accretion phase into three
regimes, to interpret the formation of the long spin period of SFXTs. The
accretion regime in some SFXT systems can be determined by their spin and
orbital periods.
