The far UV variability of the symbiotic star SY Muscae.

SY Muscae is a symbiotic star. Its periodic luminosity variation has been followed since the beginning of this century. No outburst or erratic variation has ever been seen. Its light variation is ascribed to eclipses within a binary system. We present FUV observations taken by IUE from 1981 to 1993. These spectra confirm the binary interpretation. They beautifully show the effects of Rayleigh scattering of the light from a hot source by the wind of the cool giant. The nebula within the system is strongly particle bounded. We determine the temperature and luminosity of the hot component as T^*^=~105000K, L=~1600L_sun_, corresponding to R^*^=~0.12R_sun_. We show that during the eclipse of the hot star by the red giant the nebular emission is strongly eclipsed as well. We have also analyzed the visual light curve. We determine the ephemeris from 22 cycles of visual monitoring, containing more than 2500 observations running up to 1993. The resulting period of 624.5 days is practically identical with the 625 days found by Uitterdijk from observations prior to 1934. Thus within the accuracy of the observations no change in the binary period has occurred within the last 80 years.