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We report differential photometric observations and radial-velocitymeasurements of the detached, 1.69-day period, double-lined eclipsing binary AQSer. Accurate masses and radii for the components are determined to better than1.8% 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. Thetemperatures are 6340 +/- 100 K (spectral type F6) and 6430 +/- 100 K (F5),respectively. Both stars are considerably evolved, such that predictions fromstellar evolution theory are particularly sensitive to the degree of extramixing above the convective core (overshoot). The component masses aredifferent enough to exclude a location in the H-R diagram past the point ofcentral hydrogen exhaustion, which implies the need for extra mixing. Moreover,we find that current main-sequence models are unable to match the observedproperties at a single age even when allowing the unknown metallicity, mixinglength parameter, and convective overshooting parameter to vary freely andindependently for the two components. The age of the more massive star appearssystematically younger. AQ Ser and other similarly evolved eclipsing binariesshowing the same discrepancy highlight an outstanding and largely overlookedproblem with the description of overshooting in current stellar theory.

Absolute properties of the eclipsing binary system AQ Serpentis: A stringent test of convective core overshooting in stellar evolution models