A Be star is a rapidly rotating B-type star that is ejecting a decretion (outflowing) Keplerian disk.
Be/X-ray binaries consist of a Be star in a binary with a companion that is most often a neutron star. X-ray outbursts are observed when material flows on to the neutron star. The disk dynamics in Be/X-ray binaries have some similarities with cataclysmic variables (CVs) since their binary mass ratios are comparable. However, the typically eccentric and misaligned binary orbit leads to many dynamical disk instabilities in Be/X-ray binaries that are not present in CVs. Depending upon the binary orbital parameters and disk properties, possible instabilities include superhump behavior, tilting, nodal precession, warping, Kozai-Lidov oscillations, forced eccentricity growth and eccentric disk breaking. The injection of material into the disk at the stellar equator also affects the disk dynamics. In short-orbital period and high eccentricity binaries, a radially narrow disk can remain locked to the stellar equator, though it may still undergo nodal precession if the star undergoes stellar spin-axis precession. In long-orbital-period binaries with low eccentricity, a radially extended disk can be subject to warping or nodal disk breaking. When the disk breaks, the inner ring remains locked to the Be star equator and the outer disk ring can undergo the dynamical disk instabilities. The complex behavior of Be star disks may explain the observed diversity of light curves and X-ray outbursts.