RX J0123.4-7321, a Be/X-ray binary in the wing of the SMC

Aims. To confirm faint Be/X-ray binary candidates from the XMMNewton survey of the Small Magellanic Cloud, we searched for X-ray outbursts in archival ROSAT observations. We found th at RX J0123.4-7321 was much brighter when detected with ROSA T than seen 16 years later by XMMNewton. Methods. We analysed the ROSAT observations and the OGLE I-band light curve of the optical counterpart to investigate the nature of the system. Results. High long-term variability in the X-ray flux of a factor of ∼150 was found between the ROSAT and XMMNewton detections, indicating strong outburst activity during the ROSAT obser vations. TheI-band light curve reveals long-term variability and regula r outbursts with a period of (119.9 ±2.5) days indicating the orbital period of the binary system . Conclusions. The large X-ray flux variations and the properties of the opti cal counterpart confirm RX J0123.4-7321 as a new Be /X-ray binary in the wing of the Small Magellanic Cloud.