On some features of free precession of a triaxial body: the case of Her   X-1
  We show that the free precession of a triaxial body can naturally explain the anomalously rapid change of the X-ray pulse profile of Her X-1 observed by the HEAO-1 in September 1978 without requiring a large change in the moment of inertia. 
N.I.Shakura, K.A.Postnov, M.E.Prokhorov (Sternberg Astronomical   Institute Moscow State University, Russia)