The Eclipsing Binary V1061 Cygni: Confronting Stellar Evolution Models for Active and Inactive Solar-Type Stars

We present spectroscopic and photometric observations of the eclipsing system V1061 Cyg (P=2.35 days). A third star is visible in the spectrum, and the system is a hierarchical triple. We combine the radial velocities for the three stars, times of eclipse, and intermediate astrometric data from the Hipparcos mission (abscissa residuals) to establish the elements of the outer orbit, which is eccentric and has a period of 15.8 yr. We determine accurate values for the masses, radii, and effective temperatures of the binary components: M_Aa=1.282+/-0.015 M_solar, R_Aa=1.615+/-0.017 R_solar, and T^Aa_eff=6180+/-100 K for the primary (star Aa), and M_Ab=0.9315+/-0.0068 M_solar, R_Ab=0.974+/-0.020 R_solar, and T^Ab_eff=5300+/-150 K for the secondary (Ab). The mass of the tertiary is determined to be M_B=0.925+/-0.036 M_solar and its effective temperature is T^B_eff=5670+/-150 K. Current stellar evolution models agree well with the properties of the primary but show a very large discrepancy in the radius of the secondary, in the sense that the predicted values are ~10% smaller than observed (a ~5 σ effect). In addition, the temperature is cooler than predicted, by some 200 K. These discrepancies are quite remarkable given that the star is only 7% less massive than the Sun, the calibration point of all stellar models. We identify the chromospheric activity as the likely cause of the effect. Inactive stars agree very well with the models, while active ones such as V1061 Cyg Ab appear systematically too large and too cool.