  Thanks to its long and rich observational history and rapid apsidal motion,
the massive eclipsing binary Y Cyg represents one of the cornestones to
critical tests of stellar evolution theory for massive stars. Yet, 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 analyze all these data simultaneously with the new dedicated series of
our own spectral and photometric observations from observatories widely
separated in longitude. We obtained new series of UBV 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 UBV system using the HEC22 program. We
also obtained new series of red spectra secured at two observatories and
re-analyzed earlier obtained blue electronic spectra. Our analyses provide the
most accurate so far published value of the apsidal period of 47.805 +/- 0.030
yrs and the following physical elements: M1=17.72+/-0.35$ Msun, M2=17.73+/-0.30
Msun, R1=5.785+/-0.091 Rsun, and R2=5.816+/-0.063 Rsun. 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 k2 = -1.937.
A comparison with Claret's stellar interior models implies an age close to 2
millions yrs for both stars. The claimed accuracy of modern element
determination of 1-2 per cent seems still a bit too optimistic and obtaining
new high-dispersion and high-resolution spectra is desirable.
