Context :

Aims : We present the results of the combined photometric and spectroscopic analysis of a bright ( V = 9.14 ) , nearby ( 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 in order 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 spectra with the Spectroscopy Made Easy ( SME ) package .

Results : Our analysis shows that AK For consists of two active , cool dwarfs having masses of M _ { 1 } = 0.6958 \pm 0.0010 and M _ { 2 } = 0.6355 \pm 0.0007 M _ { \odot } and radii of R _ { 1 } = 0.687 \pm 0.020 and R _ { 2 } = 0.609 \pm 0.016 R _ { \odot } , 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 rotational velocities one of the lowest , which allows us to obtain very precise radial velocity measurements .
The precision in physical parameters 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 _ { \odot } , and between short and long-period systems .
All this makes AK For a unique benchmark for understanding the properties of low-mass stars .