  Be stars are surrounded by outflowing circumstellar matter structured in the
form of decretion discs. They are often members of binary systems, where it is
expected that the decretion disc interacts both radiatively and gravitationally
with the companion. In this work we study how various orbital (period, mass
ratio and eccentricity) and disc (viscosity) parameters affect the disc
structure in coplanar systems. We simulate such binaries with the use of a
smoothed particle hydrodynamics code. The main effects of the secondary on the
disc are its truncation and the accumulation of material inwards of truncation.
We find two cases with respect to the effects of eccentricity: (i) In circular
or nearly circular prograde orbits, the disc maintains a rotating, constant in
shape, configuration, which is locked to the orbital phase. The disc is smaller
in size, more elongated and more massive for low viscosity parameter, small
orbital separation and/or high mass ratio. (ii) Highly eccentric orbits are
more complex, with the disc structure and total mass strongly dependent on the
orbital phase and the distance to the secondary. We also study the effects of
binarity in the disc continuum emission. Since the infrared and radio SED are
sensitive to the disc size and density slope, the truncation and matter
accumulation result in considerable modifications in the emergent spectrum. We
conclude that binarity can serve as an explanation for the variability
exhibited in observations of Be stars, and that our model can be used to detect
invisible companions.
