Context :

Aims : 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 .

Methods : 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 .

Results : The masses and radii are found to be M _ { 1 } = 1.1544 \pm 0.0043 M _ { \sun } , R _ { 1 } = 1.833 \pm 0.023 R _ { \sun } and M _ { 2 } = 0.7833 \pm 0.0028 M _ { \sun } , R _ { 2 } = 0.7286 \pm 0.0081 R _ { \sun } for the primary and secondary , respectively .
The effective temperatures were found to be T _ { 1 } = 6330 \pm 50 K and T _ { 2 } = 5400 \pm 80 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 .

Conclusions : 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 makes it possible to measure a precise age for the stars in this binary system and to estimate their helium abundance .
Further work is needed to explore how this helium abundance estimate depends on other free parameters in the stellar models .