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 % of the total light , which remains the main limitation to measuring the system ’ s characteristics .
We determine improved physical properties , finding masses 13.1 \pm 0.3 and 7.8 \pm 0.1 { M } _ { \odot } , radii 16.2 \pm 0.3 and 4.06 \pm 0.08 { R } _ { \odot } , and effective temperatures 21 750 \pm 280 and 21 600 \pm 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 } _ { \odot } exists and remains unexplained .