The Be/X-ray binary LS 992/RX J0812.4-3114: Physical parameters and long-term variability

We present the first long-term optical and infrared study of the optical co unterpart to the source RX J0812.4-3114, an X-ray pulsar with a Be type com panion. During the period covered by the observations the profile of some B almer lines changed from absorption to emission and back again to absorptio n. Contemporaneously, the infrared magnitudes varied by more than 0.8 mag. This long-term variability is interpreted as the formation and subsequent d issipation of the Be star's disc. The building up of the disc ended up in a n active X-ray state characterised by regular outbursts occurring at 80 day intervals. The overall duration of the formation/dissipation of the disc i s found to be less than or similar to4.3 years. Optical spectroscopic and i nfrared photometric observations were used to refine the spectral type of t he primary (B0.2IV) and to monitor the circumstellar envelope around the Be star. UBVRI and u upsilon by beta photometric observations allowed the det ermination of the astrophysical parameters of the optical companion.