Context : Rapid advancements in light-curve and radial-velocity curve modelling , as well as improvements in the accuracy of observations , allow more stringent tests of the theory of stellar evolution .
Binaries with rapid apsidal advance are particularly useful in this respect since the internal structure of the stars can also be tested .

Aims : Thanks to its long and rich observational history and rapid apsidal motion , the massive eclipsing binary Y Cyg represents one of the cornerstones of critical tests of stellar evolutionary theory for massive stars .
Nevertheless , the determination of the basic physical properties is less accurate than it could be given the existing number of spectral and photometric observations .
Our goal is to analyse all these data simultaneously with the new dedicated series of our own spectral and photometric observations from observatories widely separated in longitude .

Methods : We obtained new series of U B { } V observations at three observatories separated in local time to obtain complete light curves of Y Cyg for its orbital period close to 3 days .
This new photometry was reduced and carefully transformed to the standard U B { } V system using the HEC22 program .
We also obtained new series of red spectra secured at two observatories and re-analysed earlier obtained blue electronic spectra .
Reduction of the new spectra was carried out in the IRAF and SPEFO programs .
Orbital elements were derived independently with the FOTEL and PHOEBE programs and via disentangling with the program KOREL .
The final combined solution was obtained with the program PHOEBE .

Results : Our analyses provide the most accurate value of the apsidal period of ( 47.805 \pm 0.030 ) yrs published so far and the following physical elements : M _ { 1 } = 17.72 \pm 0.35 M _ { \odot } , M _ { 2 } = 17.73 \pm 0.30 M _ { \odot } , R _ { 1 } = 5.785 \pm 0.091 R _ { \odot } , and R _ { 2 } = 5.816 \pm 0.063 R _ { \odot } .
The disentangling thus resulted in the masses , which are somewhat higher than all previous determinations and virtually the same for both stars , while the light curve implies a slighly higher radius and luminosity for star 2 .
The above empirical values imply the logarithm of the internal structure constant \log~ { } k _ { 2 } = -1.937. A comparison with Claret ’ s stellar interior models implies an age close to 2 \times 10 ^ { 6 } yrs for both stars .

Conclusions : The claimed accuracy of modern element determination of 1–2 per cent still seems a bit too optimistic and obtaining new high-dispersion and high-resolution spectra is desirable .