Abstract
ROTSE IIId (The Robotic Optical Transient Experiment) observations of X-ray
binary system 4U 0115+634/V635 Cas obtained during 2004 June and 2005 January
make possible, for the first time, to study the correlation between optical and
type II X-ray outbursts. The X-ray outburst sharply enhanced after periastron
passage where the optical brightness was reduced by 0.3 magnitude for a few
days. We interpret the sharp reduction of optical brightness as a sign of mass
ejection from the outer parts of the disc of the Be star. After this sharp
decrease, the optical brightness healed and reached the pre X-ray outburst
level. Afterwards, gradual decrease of the optical brightness followed a
minimum then a gradual increase started again. Qualitatively, change of optical
lightcurve suggests a precession of the Be star disc around a few hundred days.
We also investigate the periodic signatures from the archival RXTE-ASM (Rossi
X-ray Timing Explorer - All Sky Monitor) light curve covering a time span of
$\sim 9$ years. We find significant orbital modulation in the ASM light curve
during the type I X-ray outburst.