2000A&A...353..952M

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 m_f=0.039M_☉. 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 M_c=1.7M_☉. Combining this value with m_f results in a mass of M_h=0.60M_☉ for the hot star. With a binary separation of 1.95AU 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.