Context : The eclipsing binary GU Mon is located in the star-forming cluster Dolidze 25 , which has the lowest metallicity measured in a Milky Way young cluster .

Aims : GU Mon has been identified as a short-period eclipsing binary with two early B-type components .
We set out to derive its orbital and stellar parameters .

Methods : We present a comprehensive analysis , including B and V light curves and 11 high-resolution spectra , to verify the orbital period and determine parameters .
We use the stellar atmosphere code fastwind to obtain stellar parameters and create templates for cross-correlation .
We obtain a model to fit the light and radial-velocity curves using the Wilson-Devinney code iteratively and simultaneously .

Results : The two components of GU Mon are identical stars of spectral type B1 V , with the same mass and temperature .
The lightcurves are typical of an EW-type binary .
The spectroscopic and photometric analyses agree on a period of 0.896640 \pm 0.000007 d. We determine a mass of 9.0 \pm 0.6 \ > M _ { \sun } for each component and temperatures of 28 000 \pm 2 000 K. Both values are consistent with the spectral type .
The two stars are overfilling their respective Roche lobes , sharing a common envelope , and therefore the orbit is synchronised and circularised .

Conclusions : The GU Mon system has a fill-out factor above 0.8 , containing two dwarf B-type stars on the main sequence .
The two stars are in a very advanced stage of interaction , with their extreme physical similarity likely due to the common envelope .
The expected evolution of such a system will very probably lead to a merger while still on the main sequence .