Context : Massive , eclipsing , double-lined , spectroscopic binaries are not common but are necessary to understand the evolution of massive stars as they are the only direct way to determine stellar masses .
They are also the progenitors of energetic phenomena such as X-ray binaries and \gamma -ray bursts .

Aims : We present a photometric and spectroscopic analysis of the candidate binary system Cyg OB2-B17 to show that it is indeed a massive evolved binary .

Methods : We utilise V band and white-light photometry to obtain a light curve and period of the system , and spectra at different resolutions to calculate preliminary orbital parameters and spectral classes for the components .

Results : Our results suggest that B17 is an eclipsing , double-lined , spectroscopic binary with a period of 4.0217 \pm 0.0004 days , with two massive evolved components with preliminary classifications of O7 and O9 supergiants .
The radial velocity and light curves are consistent with a massive binary containing components with similar luminosities , and in turn with the preliminary spectral types and age of the association .

Conclusions :