  When applied to Be/X-ray binaries, the viscous decretion disc model for Be
stars naturally predicts the truncation of the circumstellar disc. The distance
at which the circumstellar disc is truncated depends mainly on the orbital
parameters and the viscosity. In systems with low eccentricity, the disc is
expected to be truncated at the 3:1 resonance radius, for which the gap between
the disc outer radius and the critical lobe radius of the Be star is so wide
that, under normal conditions, the neutron star cannot accrete enough gas at
periastron passage to show Type I X-ray outbursts. These systems will display
only Type II X-ray outbursts. On the other hand, in systems with high orbital
eccentricity, the disc truncation occurs at a much higher resonance radius,
which is very close to or slightly beyond the critical lobe radius at
periastron unless the viscosity is very low. In these systems, disc truncation
cannot be efficient, allowing the neutron star to capture gas from the disc at
every periastron passage and display Type I outbursts regularly. In contrast to
the rather robust results for systems with low eccentricity and high
eccentricity, the result for systems with moderate eccentricity depends on
rather subtle details. Systems in which the disc is truncated in the vicinity
of the critical lobe will regularly display Type I outbursts, whereas those
with the disc significantly smaller than the critical lobe will show only Type
II outbursts under normal conditions and temporary Type I outbursts when the
disc is strongly disturbed.
