High resolution spectroscopy of symbiotic stars V. Orbital and stellar parameters for FG Ser (AS 296)

For the eclipsing symbiotic binary FG Ser (AS 296) we have obtained a serie s of high resolution optical spectra. Combining the measured radial velocit y variations for the red giant with published eclipse photometry gives a bi nary period of 650 days. We derive the radial velocity curve of the red gia nt, yielding the orbital parameters of the system and a mass function of m( f) = 0.030 M.. we further determine the rotation velocity of the red giant, and assuming corotation derive its photospheric radius. Together with the spectral type surface temperature this yields the position of the red giant in the HR-diagram and a mass estimate of AI, = 1.7 M.. Combining this valu e with m(f) results in a mass of M-h = 0.60 M. for the hot star With a bina ry separation of 1.95 AU and a radius of the red giant of 105 R. we find th at FG Ser is a detached binary with the:red giant well inside the Roche lob e.