1. Introduction

Be/X-ray binaries form a major subset of high mass X-ray binaries. These are systems consisting of a Be star earlier than B2 and a compact star, which is mostly a neutron star. The orbit is wide and eccentric. The orbital period ranges from several tens of days to several hundred days.

A Be star has two-component envelope, a polar region and an equatorial disk region. The former consists of a low-density, fast outflow emitting UV radiation, while the latter consists of a high-density plasma rotating at near Keplerian speed. Balmer lines arise from the disk region. Some systems exhibit long-term ($1 \sim 2$yr) variations in the Balmer line profiles. These variations can be attributed to the m=1 oscillations of Be-star disks (Negueruela et al. 1998a; Clark et al. 1998).

X-ray activity of Be/X-ray binaries is due to the accretion of the envelope of the Be star onto the compact star. Be/X-ray binaries exhibit several types of X-ray activity [See Stella et al. (1986) for details. See also Negueruela et al. (1998a)], which would result from a complicated interaction between the Be star envelope and the compact star.

For Be/X-ray biraries, the maximum equivalent width of the H$\alpha$line correlates with the orbital period of the system (Reig et al. 1997). This indicates that the equatorial disks around Be stars in Be/X-ray binaries are truncated by something related to the binary separation.