  The eclipsing and double-lined spectroscopic binary V380 Cyg is an extremely
important probe of stellar evolution: its primary component is a high-mass star
at the brink of leaving the main sequence whereas the secondary star is still
in the early part of its main sequence lifetime. We present extensive
high-resolution echelle and grating spectroscopy from Ondrejov, Calar Alto,
Victoria and La Palma. We apply spectral disentangling to unveil the individual
spectra of the two stars and obtain new spectroscopic elements. The secondary
star contributes only about 6% of the total light, which remains the main
limitation to measuring the system's characteristics. We determine improved
physical properties, finding masses 13.1 +/- 0.3 and 7.8 +/- 0.1 M_sun, radii
16.2 +/- 0.3 and 4.06 +/- 0.08 R_sun, and effective temperatures 21750 +/- 280
and 21600 +/- 550 K, for the primary and secondary components respectively. We
perform a detailed abundance analysis by fitting non-LTE theoretical line
profiles to the disentangled spectrum of the evolved primary star, and reveal
an elemental abundance pattern reminiscent of a typical nearby B star. Contrary
to the predictions of recent theoretical evolution models with rotational
mixing, no trace of abundance modifications due to the CNO cycle are detected.
No match can be found between the predictions of these models and the
properties of the primary star: a mass discrepancy of 1.5 M_sun exists and
remains unexplained.
