  We report on the analysis of the spin evolution of a slow X-ray pulsar GX
301$-$2 along the orbit using long-term monitoring by Fermi/GBM. Based on the
observationally confirmed accretion scenario and an analytical model for the
accretion of angular momentum we demonstrate that in this system, the neutron
star spins retrogradely, that is, in a direction opposite to the orbital
motion. This first-of-a-kind discovery of such a system proves the principal
possibility of retrograde rotation in accreting systems with suitable accretion
torque, and might have profound consequences for our understanding of the spin
evolution of X-ray pulsars, estimates of their initial spin periods, and the
ultimate result of their evolution.
