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Superorbital periods that are observed in the brightness of Be/X-ray binariesmay be driven by a misaligned and precessing Be star disc. We examine how theprecessing disc model explains the superorbital variation of (i) the magnitudeof the observed X-ray outbursts and (ii) the observed colour. Withhydrodynamical simulations we show that the magnitude of the average accretionrate on to the neutron star, and therefore the X-ray outbursts, can vary byover an order of magnitude over the superorbital period for Be star spin-orbitmisalignments $\gtrsim 70^\circ$ as a result of weak tidal truncation. MostBe/X-ray binaries are redder at optical maximum when the disc is viewed closestto face-on since the disc adds a large red component to the emission. However,A0538-66 is redder at optical minimum. This opposite behaviour requires anedge-on disc at optical minimum and a radially narrow disc such that it doesnot add a large red signature when viewed face-on. For A0538-66, themisalignment of the disc to the binary orbit must be about $70-80^\circ$ andthe inclination of the binary orbit to the line of sight must be similarlyhigh, although restricted to $<75^\circ$ by the absence of X-ray eclipses.

Disc precession in Be/X-ray binaries drives superorbital variations of outbursts and colour