  We report differential photometric observations and radial-velocity
measurements of the detached, 1.69-day period, double-lined eclipsing binary AQ
Ser. Accurate masses and radii for the components are determined to better than
1.8% and 1.1%, respectively, and are M1 = 1.417 +/- 0.021 MSun, M2 = 1.346 +/-
0.024 MSun, R1 = 2.451 +/- 0.027 RSun, and R2 = 2.281 +/- 0.014 RSun. The
temperatures are 6340 +/- 100 K (spectral type F6) and 6430 +/- 100 K (F5),
respectively. Both stars are considerably evolved, such that predictions from
stellar evolution theory are particularly sensitive to the degree of extra
mixing above the convective core (overshoot). The component masses are
different enough to exclude a location in the H-R diagram past the point of
central hydrogen exhaustion, which implies the need for extra mixing. Moreover,
we find that current main-sequence models are unable to match the observed
properties at a single age even when allowing the unknown metallicity, mixing
length parameter, and convective overshooting parameter to vary freely and
independently for the two components. The age of the more massive star appears
systematically younger. AQ Ser and other similarly evolved eclipsing binaries
showing the same discrepancy highlight an outstanding and largely overlooked
problem with the description of overshooting in current stellar theory.
