Context :

Aims : HD 166734 is an eccentric eclipsing binary system composed of two supergiant O-type stars , orbiting with a 34.5-day period .
In this rare configuration for such stars , the two objects mainly evolve independently , following single-star evolution so far .
This system provides a chance to study the individual parameters of two supergiant massive stars and to derive their real masses .

Methods : An intensive monitoring was dedicated to HD 166734 .
We analyzed mid- and high-resolution optical spectra to constrain the orbital parameters of this system .
We also studied its light curve for the first time , obtained in the VRI filters .
Finally , we disentangled the spectra of the two stars and modeled them with the CMFGEN atmosphere code in order to determine the individual physical parameters .

Results : HD 166734 is a O7.5If + O9I ( f ) binary .
We confirm its orbital period but we revise the other orbital parameters .
In comparison to what we found in the literature , the system is more eccentric and , now , the hottest and the most luminous component is also the most massive one .
The light curve exhibits only one eclipse and its analysis indicates an inclination of 63.0 \degr \pm 2.7 \degr .
The photometric analysis provides us with a good estimation of the luminosities of the stars , and therefore their exact positions in the Hertzsprung-Russell diagram .
The evolutionary and the spectroscopic masses show good agreement with the dynamical masses of 39.5 M _ { \odot } for the primary and 33.5 M _ { \odot } for the secondary , within the uncertainties .
The two components are both enriched in helium and in nitrogen and depleted in carbon .
In addition , the primary also shows a depletion in oxygen .
Their surface abundances are however not different from those derived from single supergiant stars , yielding , for both components , an evolution similar to that of single stars .

Conclusions :