The eclipse of the symbiotic star SY Muscae

We present ultra-violet observations of the symbiotic star SY Mus obtained with IUE in 1996. The observations were timed to cover ingress and egress. We observe phase dependent intensity variations of the continuum and of the emission lines. These variations are caused by a wavelength dependent opac ity with the characteristics of Rayleigh scattering plus a wavelength indep endent attenuation. From the Rayleigh scattering opacity we derive the colu mn densities of neutral hydrogen as a function of orbital phase. We then ap ply inversion methods to investigate the giant's cool wind velocity profile . A new convenient analytical expression for this profile is presented and compared to other laws. The modeling of the giant's cool wind and the assum ption of a terminal velocity of 30 km s(-1), leads to a mass loss rate of ( M) over dot approximate to 1 x 10(-9) M. yr(-1) for SY Mus.