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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 échelle and grating spectroscopy from Ondřejov, 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 per cent 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 ⊙  , radii  16.2 ± 0.3  and  4.06 ± 0.08 R ⊙  , and effective temperatures  21 750 ± 280  and  21 600 ± 550 K , for the primary and secondary components, respectively. We perform a detailed abundance analysis by fitting non‐local thermodynamic equilibrium (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 ⊙   exists and remains unexplained.

Chemical evolution of high‐mass stars in close binaries – II. The evolved component of the eclipsing binary V380 Cygni