We initiated long-term optical interferometry monitoring of the diameters ofunstable yellow hypergiants (YHG) with the goal of detecting both the long-termevolution of their radius and shorter term formation related to large mass-lossevents. We observed HR5171 A with AMBER/VLTI. We also examined archivalphotometric data in the visual and near-IR spanning more than 60 years, as wellas sparse spectroscopic data. HR5171A exhibits a complex appearance. Our AMBERdata reveal a surprisingly large star for a YHG R*=1315+/-260Rsun (~6.1AU) atthe distance of 3.6+/-0.5kpc. The source is surrounded by an extendednebulosity, and these data also show a large level of asymmetry in thebrightness distribution of the system, which we attribute to a newly discoveredcompanion star located in front of the primary star. The companion's signatureis also detected in the visual photometry, which indicates an orbital period ofPorb=1304+/-6d. Modeling the light curve with the NIGHTFALL program providesclear evidence that the system is a contact or possibly over-contact eclipsingbinary. A total current system mass of 39^{+40}_{-22} solar mass and a highmass ratio q>10 is inferred for the system. The low-mass companion of HR5171 Ais very close to the primary star that is embedded within its dense wind. Tightconstraints on the inclination and vsini of the primary are lacking, whichprevents us from determining its influence precisely on the mass-lossphenomenon, but the system is probably experiencing a wind Roche-Lobe overflow. Depending on the amount of angular momentum that can be transferred to thestellar envelope, HR5171 A may become a fast-rotating B[e]/Luminous BlueVariable (LBV)/Wolf-Rayet star. In any case, HR5171 A highlights the possibleimportance of binaries for interpreting the unstable YHGs and for massive starevolution in general.
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