Abstract

Aims. We present the results of the combined photometric and spectroscopic analysis of a bright (V = 9.14). close (d = 31 pc), late-type detached eclipsing binary. AK Fornacis, This P = 3.981 d system has not been previously recognised as a double lined spectroscopic binary and this is the first full physical model of this unique target. Methods. With the FEROS, CORALIE, and HARPS spectrographs. we collected a number of high resolution spectra to calculate radial velocities of both components of the binary. Measurements were done with our own disentangling procedure and the TODCOR technique, and were later combined with the photometry from the ASAS and SuperWASP archives. We also performed an atmospheric analysis of the component specua with the Spectroscopy Made Easy package. Results, Our analysis shows that AK For consists of two active cool dwarfs having masses of M-1 = 0.6958 +/- 0.0010 and M-2 = 0.6355 +/- 0.0007 M-circle dot and radii of R-1 = 0.687 +/- 0.020 and R-2 = 0.609 +/- 0.016 R-circle dot, respectively, which are slightly less metal-abundant than the Sun. Parameters of both components are well reproduced by the models. Conclusions, AK For is the brightest system among the known eclipsing binaries with K- or M-type stars, Its orbital period is one of the longest, and its rotational velocities is one of the lowest, which allows us to obtain very precise radial velocity measurements. The precision in physical parameters that we obtained places AK For among the binaries with the best mass measurements in the literature It also fills the gap in our knowledge of stars in the range of 0.5-0.8 M-circle dot and between short and long-period systems. All this makes AK For a unique benchmark for understanding the properties of low mass stars.