EUROPHYSICS LETTERS, cilt.14, sa.3, ss.275-280, 1991 (SCI-Expanded)
Accumulated gamma-ray evidence on the time variability of the pulse height ratio (PHR) of the Crab pulsar is shown to be compatible with a long-term (approximately 10.7 y) periodicity and can be interpreted as the free precession of the underlying neutron star. A prediction for the value of PHR by the coming GRO-EGRET gamm-ray observations under the present model is presented.