The 1984 eclipse of the symbiotic binary SY Muscae.

The authors present ultraviolet and optical observations of the 1984 eclipse of the symbiotic binary star SY Mus. The optical light curve shows a 627-day variation which is reflected in the intensity of the far-UV continuum (λ < 2000 Å) and in the intensities of all strong, permitted UV emission lines. This contrasts sharply with other eclipsing systems, in which some high ionization permitted lines show little evidence for large-scale variability. The behavior of the emission lines and the UV continuum is most naturally understood if the hot stellar source and a surrounding ionized nebula in the SY Mus binary are eclipsed by a red-giant companion every 627 days. The depth of the eclipse in the He II λ1640 emission line allows estimating the radius of the partially eclipsed He⁺ region (75 R_sun;) and that of the cool giant (60 R_sun;), for a distance of 1.3 kpc.