  4u 0115+63 is one of the most active and best studied Be/X-ray transients.
Previous studies of 4u0115+63 have led to the suggestion that it undergoes
relatively fast quasi-cyclic activity. However, due to the lack of good
coverage of the observations, the variability time scales are uncertain. Our
objective is to investigate the long-term behaviour of 4u 0115+63 to confirm
its quasi-cyclic nature and to explain its correlated optical/IR and X-ray
variability. We have performed optical/IR photometric observations and optical
spectroscopic observations of 4u 0115+63 over the last decade with
unprecedented coverage. We have focused on the Halpha line variability and the
long-term changes of the photometric magnitudes and colours and investigated
these changes in correlation with the X-ray activity of the source. results The
optical and infrared emission is characterised by cyclic changes with a period
of ~ 5 years. This long-term variability is attributed to the state of the
circumstellar disc around the Be star companion. Each cycle involves a low
state when the disc is very weak or absent and the associated low amplitude
variability is orbitally modulated and a high state when a perturbed disc
precesses, giving rise to fast and large amplitude photometric changes. X-ray
outbursts in 4u 0115+63 come in pairs, i.e., two in every cycle. However,
sometimes the second outburst is missing. Our results can be explained within
the framework of the decretion disc model. The neutron star acts as the
perturbing body, truncating and distorting the disc. The first outburst would
occur before the disc is strongly perturbed. The second outburst leads to the
dispersal of the disc and marks the end of the perturbed phase.
