Rapid multiperiodic variability in a Small Magellanic Cloud high-mass X-ray binary

The positions of high-mass X-ray binaries are often known precisely enough to identify the optical component unambiguously, and a number of those stars are or have been monitored by the Optical Gravitational Lensing Experiment (OGLE) and MACHO collaborations. The light curves of two such candidates are examined for evidence of Be-star behaviour and for periodicity. These stars are likely companions to long-period X-ray pulsars in the Small Magellanic Cloud. One of the stars exhibits two periods of 6.832 and 15.587 h, much shorter and more stable than periods of Be/X-ray binaries that are attributed to the disc of the Be star, but consistent with short-term Be variability attributed to pulsations. The multiperiodicity is quantified with Fourier techniques and examined for phase stability. The phase of both variations has wandered by more than a cycle during the 11 years of optical monitoring. Stellar pulsation is the most likely physical mechanism.