We present spectroscopic and photometric observations of the eclipsing system V1061 Cyg , previously thought to be a member of the rare class of “ cool Algols ” .
We show that it is instead a hierarchical triple system in which the inner eclipsing pair ( with P = 2.35 days ) is composed of main-sequence stars and is well detached , and the third star is also visible in the spectrum .
We combine the radial velocities for the three stars , times of eclipse , and intermediate astrometric data from the HIPPARCOS mission ( abscissae 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 \pm 0.015 M _ { \sun } , R _ { Aa } = 1.615 \pm 0.017 R _ { \sun } , and T _ { eff } ^ { Aa } = 6180 \pm 100 K for the primary ( star Aa ) , and M _ { Ab } = 0.9315 \pm 0.0068 M _ { \sun } , R _ { Ab } = 0.974 \pm 0.020 R _ { \sun } , and T _ { eff } ^ { Ab } = 5300 \pm 150 K for the secondary ( Ab ) .
The masses and radii have relative errors of only 1–2 % .
Both stars are rotating rapidly ( v \sin i values are 36 \pm 2 km s ^ { -1 } and 20 \pm 3 km s ^ { -1 } ) and have their rotation synchronized with the orbital motion .
There are signs of activity including strong X-ray emission and possibly spots .
The mass of the tertiary is determined to be M _ { B } = 0.925 \pm 0.036 M _ { \sun } and its effective temperature is T _ { eff } ^ { B } = 5670 \pm 150 K. The system is at a distance of 166.9 \pm 5.6 pc .
Current stellar evolution models that use a mixing length parameter \alpha _ { ML } appropriate for the Sun 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 \sim 10 % smaller than observed ( a \sim 5 \sigma 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 .
Similar differences have been seen before for later-type stars , but the source of the problem has remained unclear .
A comparison with the properties of other stars of similar mass as the secondary in V1061 Cyg has allowed us to 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 .
Theory provides an understanding of this in terms of the strong magnetic fields commonly associated with stellar activity , which tend to inhibit convective heat transport .
The reduced convection explains why fits to models with a smaller mixing length parameter of \alpha _ { ML } = 1.0 seem to give better agreement with the observations for V1061 Cyg Ab .