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We present multi-wavelength long-term monitoring observations of V635 Cas,the optical counterpart to the transient X-ray pulsar 4U0115+63. The evolutionof emission lines and photometric magnitudes indicates that the Be starundergoes relatively fast (~ 3-5 yr) quasi-cyclic activity, losing andreforming its circumstellar disc. We show that the general optical, infraredand X-ray behaviour can be explained by the dynamical evolution of the viscouscircumstellar disc around the Be star. After each disc-loss episode, the discstarts reforming and grows until it reaches the radius at which the resonantinteraction of the neutron star truncates it. At some point, the disc becomesunstable to (presumably radiative) warping and then tilts and startsprecessing. The tilting is very large and disc precession leads to a successionof single-peaked and shell profiles in the emission lines. Type II X-rayoutbursts take place after the disc has been strongly disturbed and wespeculate that the distortion of the disc leads to interaction with theorbiting neutron star.  We discuss the implications of these correlated optical/X-ray variations forthe different models proposed to explain the occurrence of X-ray outbursts inBe/X-ray binaries. We show that the hypothesis of mass ejection events as thecause of the spectacular variability and X-ray outbursts is unlikely to bemeaningful for any Be/X-ray binary.Comment: 16 pages, 9 figures. Accepted for publication in Astronomy &  Astrophysic

The Be/X-ray transient 4U 0115+63/V635 Cassiopeiae