Correlated V/R and infrared photometric variations in the Be/X-ray binary LS I+61 degrees 235/RX J0146.9+6121

We report on the long-term variability of the Be/X-ray binary LSI+61 degree s 235/RX J0146.9+6121. New optical spectroscopic and infrared photometric o bservations confirm the presence of global one-armed oscillations in the ci rcumstellar disc of the Be star, and allow us to derive a V/R band quasi-pe riod of 1240 +/- 30 d. Pronounced shell events, reminiscent of the spectacu lar variations in Be stars, are also seen. We have found that the J, H and K infrared photometric bands vary in correlation with the spectroscopic V/R variations, implying that the one-armed disc oscillations are prograde. Th e effect of the oscillations is not only seen in the H alpha line but is al so seen in the He I lambda 6678 and Paschen lines. As these lines are forme d at different radii in the equatorial disc of the Be star, such effects co nfirm the global nature of the perturbation. The Keplerian disc has been fo und to be denser than the average for a sample of isolated Be stars, which may be indicative of some kind of interaction with the compact companion. F inally, from a Rossi X-ray Timing Explorer observation we derive a spin per iod of the neutron star of 1404.5 +/- 0.5 s.