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Description
We report extensive radial velocity measurements of the two giant components of the detached, 104 day period binary system of Capella. Our highly accurate three-dimensional orbital solution based on all existing spectroscopic and astrometric observations including our own yields much improved masses for the primary and secondary of 2.466+/-0.018M_{sun}_ and 2.443+/-0.013M_{sun}_, with relative errors of only 0.7% and 0.5%, respectively. The mass ratio is considerably closer to unity than previously believed, which has an impact on assessing the evolutionary state of the system. Improved values are presented also for the radii (11.87+/-0.56R_{sun}_ and 8.75+/-0.32R_{sun}_), effective temperatures (4920+/-70K and 5680+/-70K), and luminosities (79.5+/-4.8L_{sun}_ and 72.1+/-3.6L_{sun}_). The distance is determined to be 13.042+/-0.028pc, based on the accurate orbital parallax. The projected rotational velocities and individual rotation periods are also known. Capella is unique among evolved stars in that, in addition to all of the above, the chemical composition is known as well.
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