  We have studied the very long-term temporal properties of the optical
emission from Be X-ray binaries (BeX) in the Small Magellanic Cloud over a ~ 16
yr baseline, using light curves from the MACHO and OGLE databases. All the BeX
in our sample display superorbital variations, many of them quasi-periodic on
timescales of ~ 200-3000 d. These long-term variations are believed to be
related to the formation and depletion of the circumstellar disc around the Be
star and we compare and contrast their behaviour with that of the LMC's
prototypical BeX, A0538-66. The great majority of sources show a correlation of
outburst amplitude with brightness (the opposite to that seen in A0538-66)
although the amplitudes are mostly small (< 0.1 mag). We suggest this is an
orbital inclination effect. In addition, we have also detected many of their
optical orbital periodicities, visible as a series of precisely regular
outbursts. Furthermore, the amplitude of these periodic outbursts can vary
through the long-term superorbital cycle, and we discuss mechanisms which can
produce this effect, as well as examining an apparent correlation between these
periodicities. As a by-product of this variation survey we have compiled a list
of all the reported SMC BeX orbital and superorbital periodicities at optical
and X-ray wavelengths.
