  AI Phe is a double-lined, detached 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. Our aim is to improve the precision of
the radius measurements for the stars in AI Phe using high-precision photometry
from 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. 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. The radii are found to be
$R_1=1.835\pm0.014$ R$_{\odot}$, $R_2=2.912\pm0.014$ R$_{\odot}$ and the masses
$M_1=1.1973\pm0.0037$ M$_{\odot}$, $M_2=1.2473\pm0.0039$ M$_{\odot}$. 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 $4.39\pm0.32$ 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. 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. More binaries with
parameters measured to a similar level of precision would allow us to test for
relationships between helium abundance and mixing length.
