¹ Foundation for Research and Technology-Hellas, 711 10 Heraklion, Crete, Greece
² Physics Department, University of Crete, 710 03 Heraklion, Crete, Greece
³ SAX SDC, Agenzia Spaziale Italiana, c/o Telespazio, via Corcolle 19, 0013 1 Roma, Italy
⁴ Physics & Astronomy Department, The University, Southampton, SO17 3BJ, UK
⁵ Departamento de Astronomía y Astrofísica, Universidad de Valéncia, E-46100 Burjassot, Valéncia, Spain
⁶ Crimean Astrophysical Observatory, 334413 Nauchny, Crimea, Ukraine
⁷ National Astronomical Observatory Rozhen, P.O. Box 136, BG-4700 Smoljan, Bulgaria
⁸ Departamento de Física, Universidad de Jaén, C/Virgen de la Cabeza 2, 23071 Jaén, Spain

We report on the long-term variability of the Be/X-ray binary LS I +61° 235/RX J0146.9+6121. New optical spectroscopic and infrared photometric observations confirm the presence of global one-arm oscillations in the circumstellar disc of the Be star and allow us to derive a V/R quasi-period of 1240±30 days. Pronounced shell events, reminiscent of the spectacular 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. The effect of the oscillations is not only seen in the H line but also in the He I 6678 and Paschen lines. Since these lines are formed at different radii in the equatorial disc of the Be star, such effect confirms the global nature of the perturbation. The Keplerian disc has been found to be denser than the average of a sample of isolated Be stars, which may be indicative of some kind of interaction with the compact companion. Finally, from a Rossi X-ray Timing Explorer observation we derive a spin period of the neutron star of 1404.5±0.5 s.

Accepted by MNRAS
Preprints from pablo@physics.uoc.gr
or on the web at http://xxx.lanl.gov/abs/astro-ph/0005033