We present our final orbit for the late-type spectroscopic binary HD 1. Employed are 553 spectra from 13 years of observations with our robotic STELLA facility and its high-resolution echelle spectrograph SES. Its long-term radial-velocity stability is ≈50m/s . A single radial velocity of HD 1 reached a rms residual of 63m/s, close to the expected precision. Spectral lines of HD 1 are rotationally broadened with vsini of 9.1±0.1 km/s . The overall spectrum appears single-lined and yielded an orbit with an eccentricity of 0.5056±0.0005 and a semi-amplitude of 4.44km/s . We constrain and refine the orbital period based on the SES data alone to 2318.70±0.32d, compared to 2317.8±1.1d when including the older data set published by DAO and Cambridge/Coravel. Owing to the higher precision of the SES data, we base the orbit calculation only on the STELLA/SES velocities in order not to degrade its solution. We redetermine astrophysical parameters for HD 1 from spectrum synthesis and, together with the new {\it Gaia} DR-2 parallax, suggest a higher luminosity than published previously. We conclude that HD 1 is a slightly metal-deficient K0 III-II giant 217 times more luminous than the Sun. The secondary remains invisible at optical wavelengths. We present evidence for the existence of a third component.
@article{arxiv.2011.13330,
title = {On the binary orbit of Henry Draper one (HD 1)},
author = {Klaus G. Strassmeier and Michael Weber},
journal= {arXiv preprint arXiv:2011.13330},
year = {2021}
}