¹ SAX SDC, ASI, c/o Nuova Telespazio, via Corcolle 19, I00131 Rome, Italy
² Astrophysics Research Institute, Liverpool John Moores University, Byrom St., Liverpool, L3 3AF, UK
³ Faculty of Engineering, Hokkai-Gakuen University, Toyohira-ku, Sapporo 062-8605, Japan

We present photometry and high SNR spectroscopy in the classification region of V635 Cas, the optical counterpart to the transient X-ray pulsator 4U 0115+63, taken at a time when the circumstellar envelope had disappeared. V635 Cas is classified as a B0.2Ve star at a distance of 7-8 kpc. We use the physical parameters derived from these observations and the orbit derived from X-ray observations to elaborate a model of the system based on the theory of decretion discs around Be stars. We show that the disc surrounding the Be star must be truncated by the tidal/resonant interaction with the neutron star and cannot be in a steady state. This explains many of the observed properties of 4U 0115+63. In particular, because of this effect, under normal circumstances, the neutron star cannot accrete from the disc, which explains the lack of regular Type I outbursts from the source.

Accepted by Astronomy & Astrophysics
Preprints from ignacio@astro.u-strasbg.fr
or on the web at http://xxx.lanl.gov/abs/astro-ph/0011407