1999A&A...349..169D

For the eclipsing symbiotic binary system SY Mus we present evidence for an asymmetric density distribution around the M-giant. We obtain this result from an improved orbital solution. We determine a revised orbital period of P=625.0±0.5 d from UV eclipse observations and a re-analysis of the visual light curve. Based on new radial velocity observations, we measure the epoch of mid-eclipse accurately to within ±6 days, or 1/100 of the period. At ingress the eclipse curve is steeper and closer to mid-eclipse than at egress. From measured column densities of neutral hydrogen during egress we conclude that the mass loss rate of the M-giant is of the order of {dot}(M)≃5x10^–7M_☉/yr. The eclipse of the UV light in SY Mus and RW Hya can be partially explained by Rayleigh scattering but there is also an additional source of opacity. HST high resolution UV observations of RW Hya during egress reveal that this additional flux attenuation is due to blanketing by numerous absorption lines, predominantly FeII. Line blanketing is an important effect that has to be taken into account to explain the eclipse of SY Mus and of other symbiotic binaries.