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tokens: reported 420.8 d period obtained from analysing all the MACHO data, i.e. including the orbital outburst points. Figure 1. MACHO V (a) and R (b) light curves; (c) MACHO colour variation, (d) OGLE IV light curves together with the mean MACHO V light curve (in grey) projected forward using the ephemeris of Alcock et al. ( 2001 ). The red and blue vertical lines refer to the times of the SALT and SAAO 1.9 m telescope observations, respectively. See Fig. 10 for more detail regarding the timing of the SALT spectra within the overall OGLE IV light curve. Figure 2. Lomb–Scagle periodogram of the MACHO V data showing peaks at 445.1 d, 352.7 d and 391.4 d for the first half (a), second half (b) and complete light curve (c), respectively. (d) Power spectrum of the OGLE IV data showing a peak at 423.74 d. To further explore the variation in the time-scale of the superorbital modulation, the complete MACHO data were divided into two parts. The power spectrum of the first and second half shows a peak at 446.0 ± 0.2 d and 355.4 ± 0.29 d, respectively (Fig. 2 ). The OGLE IV also shows a long-term variation but now of a much smaller amplitude, although still with a period consistent with that in the MACHO data. When all bright points related to the 16.64 d outburst were removed (as in the MACHO data) the power spectrum shows its highest peak at P sup = 423.74 ± 0.42 d and some additional peaks around 200 d (see Fig. 2 ). We have compared this behaviour to the original superorbital period by projecting the 420.8 d light curves and ephemeris by Alcock et al. ( 2001 ) into the OGLE IV data (Fig. 1 ). The optical maxima in the MACHO data are flat (i.e. no optical outbursts were seen) and last ∼200 d, meaning that we are seeing the completely disc-less B star. However in the OGLE IV phase, the optical maxima are not flat and the orbital outbursts now occur throughout the OGLE superorbital cycle, implying that during the OGLE IV optical maxima the equatorial disc was not completely dissipated. The MACHO and OGLE IV light curves folded on the superorbital period are shown in Fig. 4 . The colour variation ( V − R ) of A0538–66 shows that the source is bluer at optical maxima and becomes redder as the source fades. This behaviour is opposite to that seen in most other BeX where the source is redder when it is brighter. However, during outbursts on the orbital period associated with periastron passage of the NS, the optical brightening is accompanied by a slight increase in reddening (i.e. the source gets redder). The amplitude of the outbursts in the MACHO data clearly varies throughout the long-term 420.8 d cycle, being very strong (∼0.7 mag) at optical minima, decreasing in amplitude as the source brightens and completely disappearing at optical maxima. Similar behaviour is seen in the OGLE IV data; however, there are clearly orbital outbursts occurring even when the source is at maximum in the long-term cycle. In order to refine the orbital period, we removed the