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We combine the decade long photometry of the Be/X-ray binary system A0538-66provided by the MACHO and OGLE IV projects with high resolution SALTspectroscopy to provide detailed constraints on the orbital parameters andsystem properties. The ~420d superorbital modulation is present throughout, buthas reduced in amplitude in recent years. The well-defined 16.6409d orbitaloutbursts, which were a strong function of superorbital phase in the MACHO data(not occurring at all at superorbital maximum), are present throughout the OGLEIV coverage. However, their amplitude reduces during superorbital maximum. Wehave refined the orbital period and ephemeris of the optical outburst based on~25 yrs light curves to HJD = 2455674.48 +/- 0.03 + n*16.6409 +/- 0.0003d. OurSALT spectra reveal a B1 III star with vsini of 285 km/s from which we havederived an orbital radial velocity curve which confirms the high eccentricityof e = 0.72 +/- 0.14. Furthermore, the mass function indicates that, unless theneutron star far exceeds the canonical 1.44 Msun, the donor must besignificantly undermassive for its spectral type. We discuss the implicationsof the geometry and our derived orbital solution on the observed behaviour ofthe system.

Orbital and Superorbital Monitoring of the Be/X-ray binary A0538-66: constraints on the system parameters