  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.
