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Asteroseismology of solar-like oscillations in giant stars allow thederivation of their masses and radii. For members of open clusters this allowsan age estimate of the cluster which should be identical to the age estimatefrom the colour-magnitude diagram, but independent of the uncertainties thatare present for that type of analysis. Thus, a more precise and accurate ageestimate can be obtained. We aim to measure asteroseismic properties ofoscillating giant members of the open cluster NGC 6866 and utilise these for acluster age estimate. Model comparisons allow constraints on the stellarphysics, and here we investigate the efficiency of convective-core overshootand effects of rotation during the main-sequence, which has a significantinfluence on the age for these relatively massive giants. We identify six giantmembers of NGC 6866 and derive asteroseismic measurements for five of them.This constrains the convective-core overshoot and enables a more precise andaccurate age estimate than previously possible. Asteroseismology establishesthe helium-core burning evolutionary phase for the giants, which have a meanmass of 2.8 $M_{\odot}$. Their radii are significantly smaller than predictedby current 1D stellar models unless the amount of convective-core overshoot onthe main sequence is reduced to $\alpha_{ov} \leq 0.1 H_p$ in thestep-overshoot description. Our measurements also suggest that rotation hasaffected the evolution of the stars in NGC 6866 in a way that is consistentwith 3D simulations but not with current 1D stellar models. The cluster age isestimated to be 0.43 $\pm$ 0.05 Gyr, significantly younger and more precisethan most previous estimates. We derive a precise cluster age whileconstraining convective-core overshooting and effects of rotation in themodels. We uncover potential biases for automated age estimates of helium-coreburning stars.

An asteroseismic age estimate of the open cluster NGC 6866 using Kepler and Gaia