Phase changes of the Be/X-ray binary X Persei

We present high resolution optical spectroscopy and V band photometry obtai ned during the period 1987-2001 for the Be star X Persei/HD 24534, the coun terpart to the X-ray pulsar 4U 0352+30. We find that throughout this interv al X Per is highly active, with significant photometric and spectroscopic v ariability. We identify one episode of complete disc loss during this perio d (1988 May-1989 June), characterised by significant DeltaV = 0.6 mag optic al fading and the presence of purely photospheric H alpha and He I 6678 Ang strom lines. Two further episodes of pronounced optical fading which did no t result in the complete dispersal of the circumstellar disc were also iden tified (1994 October-1995 October and 1999 November-present). The emission line profiles of both H alpha and He I 6678 Angstrom also show significant variability. Cyclic changes in the strength of the peaks in both emission l ines are observed, with periods ranging from 0.6-2 yrs-we attribute these t o the presence of a one armed density wave in the inner circumstellar disc. Additional structure at large projected velocities is also present in the He I line-suggesting the presence of a significant density enhancement in t he disc near the stellar surface (the "double disc" of Tarasov & Roche). Th e evolution of the outer edge of the H alpha emitting region of the circums tellar disc is followed during disc formation, and is found to increase rat her slowly. This observation, combined with the presence of the one armed d ensity wave and the rate of disc formation and loss all provide strong evid ence for the hypothesis that the circumstellar disc of X Per is a viscous d ecretion disc, with angular momentum being supplied by an as yet unknown ph ysical mechanism near the stellar surface.