Extreme variability in the Be-type, periodic recurrent X-ray transient A0538-66: a highly eccentric interacting binary

Optical spectroscopy and photometry and IUE spectroscopy of the counterpart of the extremely powerful LMC recurrent X-ray transient AO538-66 are presented. During one 16.6 day outburst cycle at the end of 1980 December the optical spectra show steadily increasing Balmer and He I emission (indicative of an expanding envelope) superposed on a B2 III-IV spectrum. There is a substantial optical brightness increase of 2 mag at the peak, accompanied by the sudden turn-on of He II lambda 4686. These and other optical spectra show significant radial velocity changes but no clear correlation with the 16.6 day cycle is found. Further optical and IUE spectra were obtained at the peak of subsequent outbursts, and these show very strong and broad (approx. 5000 km s-1) emission lines. The system is interpreted as a approx. 1 Msun compact object in a highly eccentric (e approx. 0.7) orbit about the approx. 12 Msun B star with a binary period equal to the flare period. Continuum measurements indicate that the emitting region at outburst is several times the size of the B star. (author)