BVRIJHK light curves of 4U 0115+63/V635 Cas

Previous studies of 4U 0115+63 have led to the suggestion that 4U 0115+63 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/V635 Cas 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/V635 Cas over the last decade with unprecedented coverage. We have focused on the H{alpha} 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.