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(Abridged) We present spectroscopic and photometric observations of theeclipsing system V1061 Cyg (P = 2.35 days). We show that it is a hierarchicaltriple in which the third star is visible in the spectra. We combine the radialvelocities for the three stars, times of eclipse, and intermediate astrometricdata from the HIPPARCOS mission (abscissa residuals) to establish the elementsof the outer orbit (P = 15.8 yr) and accurate values for the masses and radii(1-2% errors) and the effective temperatures of the binary components. Bothstars are rotating rapidly and have their rotation synchronized with theorbital motion. There are signs of activity including strong X-ray emission andpossibly spots. Current stellar evolution models agree well with the propertiesof the primary, but show a very large discrepancy in the radius of thesecondary: the predicted values are about 10% smaller than observed (a 5-sigmaeffect). Also, the temperature is cooler than predicted by some 200 K. Thesediscrepancies are quite remarkable given that the star is only 7% less massivethan the Sun, the calibration point of all stellar models. We identify thechromospheric activity as the likely cause of the effect. Inactive stars agreevery well with the models, while active ones such as V1061 Cyg Ab appearsystematically too large and too cool. Theory provides an understanding of thisin terms of the strong magnetic fields commonly associated with stellaractivity, which tend to inhibit convective heat transport.Comment: 58 pages, including 11 tables and 16 figures. To appear in 01 April  2006 issue of the Astrophysical Journa

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