Accurate absolute parameters of the binary system V4089 Sgr

•We took 20 high-resolution spectra of the binary V4089 Sgr.•We fitted simultaneously measured radial velocities and published photometric data.•Masses and radii of the binary components with errors below 1% were obtained.•Theoretical evolutionary models led to constrain the metallicity and age of the binary.

We carried out a spectroscopic and photometric analysis of the binary V4089 Sgr with the aim to obtain absolute masses and radii of the components and to compare these parameters with stellar evolution theoretical models. We took high-resolution spectra and measured radial velocity using standard cross-correlations and a technique of spectral disentangling. Absolute parameters of the components were determined through the simultaneous fitting of measured radial velocities and Geneva photometric data available in the literature. In this way we obtained MA=2.584±0.008M⊙,MB=1.607±0.007M⊙,RA=3.959±0.013R⊙, and RB=1.605±0.016R⊙. The comparison of these parameters with two grids of theoretical models led to estimate narrow ranges of possible values for system metallicity and age. According to circularization theory it was not expected that the binary had achieved a circular orbit as a result of tidal friction, so the null eccentricity found is an interesting fact. On the other hand, we measured the projected rotational velocity of both components and we conclude that the primary is not synchronized with orbital motion.