High resolution spectroscopy of symbiotic stars ? V . Orbital

For the eclipsing symbiotic binary FG Ser (AS 296) we have obtained a series of high resolution optical spectra. Combining the measured radial velocity variations for the red giant with published eclipse photometry gives a binary period of 650 days. We derive the radial velocity curve of the red giant, yielding the orbital parameters of the system and a mass function of mf = 0.039 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 Mc = 1.7 M . Combining this value with mf results in a mass of Mh = 0.60 M for the hot star. With a binary separation of 1.95 AU and a radius of the red giant of 105R we find that FG Ser is a detached binary with the red giant well inside the Roche lobe.