Context : AI Phe is a double-lined , detached eclipsing binary , in which a K-type sub-giant star totally eclipses its main-sequence companion every 24.6 days .
This configuration makes AI Phe ideal for testing stellar evolutionary models .
Difficulties in obtaining a complete lightcurve mean the precision of existing radii measurements could be improved .

Aims : Our aim is to improve the precision of the radius measurements for the stars in AI Phe using high-precision photometry from the Wide Angle Search for Planets ( WASP ) , and use these improved radius measurements together with estimates of the masses , temperatures and composition of the stars to place constraints on the mixing length , helium abundance and age of the system .

Methods : A best-fit ebop model is used to obtain lightcurve parameters , with their standard errors calculated using a prayer-bead algorithm .
These were combined with previously published spectroscopic orbit results , to obtain masses and radii .
A Bayesian method is used to estimate the age of the system for model grids with different mixing lengths and helium abundances .

Results : The radii are found to be R _ { 1 } = 1.835 \pm 0.014 { R } _ { \sun } , R _ { 2 } = 2.912 \pm 0.014 { R } _ { \sun } and the masses M _ { 1 } = 1.1973 \pm 0.0037 { M } _ { \sun } , M _ { 2 } = 1.2473 \pm 0.0039 { M } _ { \sun } .
From the best-fit stellar models we infer a mixing length of 1.78 , a helium abundance of Y _ { AI } = 0.26 ^ { +0.02 } _ { -0.01 } and an age of 4.39 \pm 0.32 Gyr .
Times of primary minimum show the period of AI Phe is not constant .
Currently , there are insufficient data to determine the cause of this variation .

Conclusions : Improved precision in the masses and radii have improved the age estimate , and allowed the mixing length and helium abundance to be constrained .
The eccentricity is now the largest source of uncertainty in calculating the masses .
Further work is needed to characterise the orbit of AI Phe .
Obtaining more binaries with parameters measured to a similar level of precision would allow us to test for relationships between helium abundance and mixing length .