Introduction

The X-ray transient RX J0117.6-7330 was discovered by the Position Sensitive Proportional Counter on board ROSAT on October 1-2, 1992 (Clark, Remillard & Woo 1996, 1997); it is located approximately 5' south-east of the X-ray binary SMC X-1. Its optical counterpart was observed by Charles, Southwell & O'Donoghue (1996), and by Clark et al. (1997), and has the characteristics of a Be star; the system is therefore likely to be a High-Mass X-ray Binary (HMXB).

Systems with a Be-type primary are the largest group among the HMXBs, both in the Galaxy and in the Magellanic Clouds (van Paradijs 1995; Kahabka & Pietsch 1996). In these systems, X-ray outbursts lasting from weeks to months are caused by sudden enhancements of the equatorial mass loss of the Be star; during an active state, a modulation corresponding to the orbital period is often observed, with increased X-ray emission when the compact object transits near periastron in an eccentric orbit (van den Heuvel & Rappaport 1987). Active states are separated by longer inactive periods, often lasting several years.

The nature of the compact object in RX J0117.6-7330 is not yet known: its soft X-ray spectrum, its long decay time after the 1992 outburst and the absence of pulsations make this system a possible black-hole candidate (Clark et al. 1997). A common way to determine whether the compact object in an X-ray binary can be a neutron star is to deduce its mass function from the radial velocity amplitude of the primary; such measurements are more difficult in HMXBs owing to the low velocity expected for the primary and to the long binary period (tens to hundreds of days).

© Copyright Astronomical Society of Australia 1997