Optical spectroscopy of V635 Cassiopeiae/4U 0115+63

V635 Cas is the optical counterpart of the X-ray bi- nary system 4U 0115+63. It was previously tentatively identi- fied as a Be star based on its optical colours and the presence of H emission. Our observations indicate that it is an O9e star. This is the first direct determination of this star's optical spec- tral type. The presence of a hotter companion star may in part explain the large temporal variation observed in this system. Extreme variability was observed in 1992 February when both the H and a series of Paschen lines changed from emis- sion to absorption. This was interpreted as a disk-loss event and it is the first time that it has been observed in this system. We use far red spectra of V635 Cas to probe the circumstellar disk, dis- cussing the various line formation regions. The lines observed are consistent with a late type Oe star. The flux standard Hiltner 102 was also observed. Although it is classified as a B0 III star, we re-classify it as a O9.7 II star with a slight nitrogen enhancement.