Context : Only a few stars are caught in the very brief and often crucial stages when they quickly traverse the Hertzsprung-Russell diagram , and none has yet been spatially resolved in the mass transfer phase .

Aims : We initiated long-term optical interferometry monitoring of the diameters of massive and unstable yellow hypergiants ( YHG ) with the goal of detecting both the long-term evolution of their radius and shorter term formation of a possible pseudo-photosphere related to proposed large mass-loss events .

Methods : We observed HR 5171 A with AMBER/VLTI .
We also examined archival photometric data in the visual and near-IR spanning more than 60 years , as well as sparse spectroscopic data .

Results : HR 5171 A exhibits a complex appearance .
Our AMBER data reveal a surprisingly large star for a YHG R _ { * } =1315 \pm 260R _ { \odot } Â ( or \sim 6.1 AU ) at the distance of 3.6 \pm 0.5kpc .
The source is surrounded by an extended nebulosity , and these data also show a large level of asymmetry in the brightness distribution of the system , which we attribute to a newly discovered companion star located in front of the primary star .
The companion ’ s signature is also detected in the visual photometry , which indicates an orbital period of P _ { orb } =1304 \pm 6d .
Modeling the light curve with the NIGHTFALL program provides clear evidence that the system is a contact or possibly over-contact eclipsing binary .
A total current system mass of 39 ^ { +40 } _ { -22 } M _ { \odot } and a high mass ratio q \geq 10 is inferred for the system .

Conclusions : The low-mass companion of HR 5171 A is very close to the primary star that is embedded within its dense wind .
Tight constraints on the inclination and vsini of the primary are lacking , which prevents us from determining its influence precisely on the mass-loss phenomenon , but the system is probably experiencing a wind Roche-Lobe overflow .
Depending on the amount of angular momentum that can be transferred to the stellar envelope , HR 5171 A may become a fast-rotating B [ e ] /Luminous Blue Variable ( LBV ) /Wolf-Rayet star .
In any case , HR 5171 A highlights the possible importance of binaries for interpreting the unstable YHGs and for massive star evolution in general .