Circumstellar matter around M-giants in symbiotic binaries: SY Muscae and RW Hydrae

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 t he epoch of mid-eclipse accurately to within +/-6 days, or 1/100 of the per iod. At ingress the eclipse curve is steeper and closer to mid-eclipse than at egress. From measured column densities of neutral hydrogen during egres s we conclude that the mass loss rate of the M-giant is of the order of M a pproximate to 5 . 10(-7) Mo./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 thi s additional flux attenuation is due to blanketing by numerous absorption l ines, predominantly Fe II. Line blanketing is an important effect that has to be taken into account to explain the eclipse of SY Mus and of other symb iotic binaries.