Photometry and spectroscopy of AQ Ser

Accurate masses and radii for the components are determined to better than 1.8% and 1.1%, respectively, and are M_1_=1.417+/-0.021M_{sun}_, M_2_=1.346+/-0.024M_{sun}_, R_1_=2.451+/-0.027R_{sun}_, and R_2_=2.281+/-0.014R_{sun}_. The temperatures are 6340+/-100K (spectral type F6) and 6430+/-100K (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.