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Exoplanet searches through space-based photometric time series have shown tobe very efficient in recent years. However, follow-up efforts on the detectedplanet candidates have been demonstrated to be critical to uncover the truenature of the transiting objects. In this paper we show a detailed analysis ofone of those false positives hidden as planetary signals. In this case, thecandidate KOI-3886.01 showed clear evidence of a planetary nature from varioustechniques. Indeed, the properties of the fake planet set it among the mostinteresting and promising for the study of planetary evolution as the starleaves the main sequence. To unveil the true nature of this system, we presenta complete set of observational techniques including high-spatial resolutionimaging, high-precision photometric time series (showing eclipses, phase curvevariations and asteroseismology signals), high-resolution spectroscopy andderived radial velocities, to unveil the true nature of this planet candidate.We find that KOI-3886.01 is an interesting false positive case: a hierarchicaltriple system composed by a $\sim$K2III giant star (KOI-3886A) accompanied by aclose-in eclipsing binary formed by a subgiant $\sim$G4IV star (KOI-3886B) anda brown dwarf (KOI-3886C). In particular, KOI-3886C is one of the mostirradiated brown dwarfs known to date, showing the largest radius in thissubstellar regime. It is also the first eclipsing brown dwarf known around anevolved star. In this paper we highlight the relevance of complete sets offollow-up observations to extrasolar planets detected by the transit techniqueusing large-pixel photometers such as Kepler and TESS, and in the future,PLATO. In particular, multi-color high-spatial resolution imaging was the firsthint toward ruling out the planet scenario in this system.

Uncovering the ultimate planet impostor. An eclipsing brown dwarf in a hierarchical triple with two evolved stars