  Abridged. Our aim is to measure the masses and radii of the stars in a
newly-discovered detached eclipsing binary system to a high precision
($\approx$1%), enabling the system to be used for the calibration of free
parameters in stellar evolutionary models. Photometry from the Wide Angle
Search for Planets (WASP) project was used to identify 1SWASP
J063930.33-322404.8 (TYC 7091-888-1, WASP 0369-32 hereafter) as a detached
eclipsing binary system with total eclipses and an orbital period of P=11.66
days. Lightcurve parameters for WASP 0639-32 are obtained using the ebop
lightcurve model, with standard errors evaluated using a prayer-bead algorithm.
Radial velocities were measured from 11 high-resolution spectra using a
broadening function approach, and an orbit was fitted using sbop. Observed
spectra were disentangled and an equivalent width fitting method was used to
obtain effective temperatures and metallicities for both stars. A Bayesian
framework is used to explore a grid of stellar evolution models, where both
helium abundance and mixing length are free to vary, and use observed
parameters (mass, density, temperature and metallicity) for each star to obtain
the age and constrain the helium abundance of the system. The masses and radii
are found to be $M_{1}=1.1544\pm0.0043\,M_{\odot}$,
$R_{1}=1.833\pm0.023\,R_{\odot}$ and $M_{2}=0.7833\pm0.0028\,M_{\odot}$,
$R_{2}=0.7286\pm0.0081\,R_{\odot}$ for the primary and secondary, respectively.
The effective temperatures were found to be $T_{1}=6330\pm50$ K and
$T_{2}=5400\pm80$ K for the primary and secondary star, respectively. The
system has an age of $4.2^{+0.8}_{-0.1}$ Gyr, and a helium abundance in the
range 0.251-0.271. WASP 0639-32 is a rare example of a well-characterised
detached eclipsing binary system containing a star near the main-sequence
turn-off point. This make it possible to measure a precise age for the stars
...
