Refining the parallax in visual double stars using orbital and spectral data: Application to the system of the K0 giants, A 1808

In this paper, we show how Hipparcos parallaxes of distant visual binaries (pi(Hip) less than or similar to 10 mas) can be refined. This is accomplished by using relevant orbital and spectral data. To this end, we study the pair of K0 giants, A 1808. A revised orbit of this system is presented that was calculated by taking into account a set of interferometric and visual measurements covering almost one orbital revolution. In addition, by analyzing its spectrum obtained with the 2.6 m telescope of the Byurakan Astrophysical Observatory (Armenia), we confirm the MK spectral type, K0III. On the basis of B - V photometric data, we derived the extinction A(V) = 0.23 +/- 0.08 mag as well as obtained a spectroscopic parallax (pi(sp) = 3.19 +/- 0.21 mas) with a much smaller relative uncertainty of 6.6% compared with that of Hipparcos (42.1%; pi(Hip) = 2.43 +/- 1.00 mas). Accordingly, this system is now placed at 313 +/- 21 pc. With the obtained spectroscopic parallax, a dynamical mass of 4.52(-0.90)(+1.24) M-circle dot is obtained, which agrees well with that expected from standard calibrations.