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HD 81809 has one of the highest quality activity cycles from the sample ofstars synoptically observed in the Mount Wilson Observatory HK Project.However, this object is in fact a binary system, raising the question as towhich of the components is responsible for the observed cyclic activity andwhat are the properties of that active component. The Hipparcos spacecraftobtained resolved two-color photometry for this system that indicates that bothcomponents are near the solar temperature. Combined with the precise Gaiaparallax and empirical bolometric corrections we derive component luminositiesof L_A = 5.8 +/- 0.3 Lsun and L_B = 1.025 +/- 0.055 Lsun, and radii R_A = 2.42+/- 0.08 Rsun and R_B = 1.04 +/- 0.04 Rsun, confirming that the primarycomponent is a subgiant. We perform an independent estimate of the rotationperiod of the A component based on vsini and find that it agrees with the 40.2d period previously measured from the Ca HK time series. We explore plausiblescenarios for the deconvolved S-index and find that a cycling A component wouldhave an activity level within the bounds of ensemble activity-rotation trends,while a cycling B component likely does not. Based on the available rotationand activity evidence, we find the most likely characterization of the systemis a subgiant primary component responsible for the smooth cyclic behavior inCa HK with log(R'_HK) ~ -4.89, while the secondary component has relativelyflat activity at log(R'_HK) ~ -5.02.

Deconvolving the HD 81809 binary: rotational and activity evidence for a subgiant with a Sun-like cycle