HR5171 A is a yellow hypergiant star, which was not well studied in the past. In that work, we measured the angular diameter of the star by means of optical long-baseline interferometry, far larger than expected with a 1300 solar diameter.

We also discovered a companion star, which happen to be orbiting just at the surface boundary of the huge primary star. These unique interferometric data were complemented by a wealth of additional imaging, spectrometric, and photometric measurements which happened to be consistent with the presence of the companion star.

The presence of this orbiting star could be one cause of the very unusual properties of the primary star.

This is a work led by Olivier Chesenau, in which I did some contributions to the interferometric data interpretation, especially the model-fitting part, which allowed us to find the unsuspected companion and disentangle the spectra of the different components.

It has been the subject of a nice press release from ESO, dubbed in the press in many places :