  We present the most precise to date orbital and physical parameters of the
well known short period (P=5.975 d), eccentric (e=0.3) double-lined
spectroscopic binary BY Draconis, a prototype of a class of late-type, active,
spotted flare stars. We calculate the full spectroscopic/astrometric orbital
solution by combining our precise radial velocities (RVs) and the archival
astrometric measurements from the Palomar Testbed Interferometer (PTI). The RVs
were derived based on the high resolution echelle spectra taken between 2004
and 2008 with the Keck I/HIRES, Shane/CAT/HamSpec and TNG/SARG
telescopes/spectrographs using our novel iodine-cell technique for double-lined
binary stars. The RVs and available PTI astrometric data spanning over 8 years
allow us to reach 0.2-0.5% level of precision in M sin3(i) and the parallax but
the geometry of the orbit (i=154 deg) hampers the absolute mass precision to
3.3%, which is still an order of magnitude better than for previous studies. We
compare our results with a set of Yonsei-Yale theoretical stellar isochrones
and conclude that BY Dra is probably a main sequence system more metal-rich
than the Sun. Using the orbital inclination and the available rotational
velocities of the components, we also conclude that the rotational axes of the
components are likely misaligned with the orbital angular momentum. Given BY
Dra's main sequence status, late spectral type and the relatively short orbital
period, its high orbital eccentricity and probable spin-orbit misalignment are
not in agreement with the tidal theory. This disagreement may possibly be
explained by smaller rotational velocities of the components and the presence
of a substellar mass companion to BY Dra AB.
