  The tidal interaction of a Be star with a binary companion forms two spiral
arms that cause orbital modulation of the Be disc structure. The aim of this
work is to identify observables in which this modulation is apparent. The
structure of a Be disc in a coplanar circular binary system is computed with a
smoothed-particle hydrodynamics code, and a radiation transfer code calculates
the spectral energy distribution. Line depolarisation was confirmed, with
polarisation profiles nearly reverse to emission-line profiles. The continuum
flux maximizes for pole-on discs, but photometric variability maximizes for
edge-on discs. The linear polarisation exhibits one or two maxima per orbital
cycle. While polarisation variability in visible passbands is important only at
low inclinations, infrared bands may demonstrate high orbital variability even
at large inclinations. More evident is the modulation in the polarisation angle
(PA) for low inclinations. The latter can be used to track azimuthal
asymmetries for pole-on discs, where the spectroscopic variability in the
violet-to-red (V/R) emission-component ratio disappears. PA reversals coincide
with phases where V/R=1, tracking lines of sight directed towards regions where
the approaching and receding arms overlap. Continuum flux and polarisation are
mostly in phase for neighbouring wavelength regions. It is suggested that
studies of non-symmetric discs distorted by tidal forces from a secondary star
may be used to study disc variabilities of other origins.
