  We present the first long-term optical and infrared study of the optical
counterpart to the source RX J0812.4-3114, an X-ray pulsar with a Be type
companion. During the period covered by the observations the profile of some
Balmer lines changed from absorption to emission and back again to absorption.
Contemporaneously, the infrared magnitudes varied by more than 0.8 mag. This
long-term variability is interpreted as the formation and subsequent
dissipation of the Be star's disc. The building up of the disc ended up in an
active X-ray state characterised by regular outbursts occurring at 80 day
intervals. The overall duration of the formation/dissipation of the disc is
found to be < 4.3 years. Optical spectroscopic and infrared photometric
observations were used to refine the spectral type of the primary (B0.2IV) and
to monitor the circumstellar envelope around the Be star. UBVRI and uvby\beta
photometric observations allowed the determination of the astrophysical
parameters of the optical companion.

Astrophysics