Long term spectral variability of Hen 3-640 (A1118-61)

We present spectroscopic observations of the peculiar emission line star He n 3-640, the optical counterpart of the hard X-ray source A 1118-61, from 1 985 to 1997. Our data confirm strong spectral variability (both on long and short time scales) of the star and the correlation between the hard X-ray flux and the Ha equivalent width. We also found a significant H alpha/H bet a flux ratio variability during the last 12 years and a clear evidence of a general decrease of the ionization level after the 1992 outburst. The deri ved scenario is that of a large extended envelope surrounding Hen 3-640, ex panding because of processes intrinsic to the optical star; when the densit y around the compact object reaches a value large enough to generate a stea dy accretion disk around the A1118-61 pulsar, the mass transfer efficiency and thus the X-ray flux sharply increases. Our scenario does not foreseen a ny exact periodicity in the X-ray outbursts of the systems.