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Using high-contrast imaging with the SPHERE instrument at the VLT, we reportthe first images of a cold brown dwarf companion to the exoplanet host starHD4113A. The brown dwarf HD4113C is part of a complex dynamical systemconsisting of a giant planet, stellar host and a known wide M-dwarf companion.Its separation of $535\pm3$mas and H-band contrast of $13.35\pm0.10$magcorrespond to a projected separation of 22AU and an isochronal mass estimate of$36\pm5$M$_J$ based on COND models. The companion shows strong methaneabsorption, and through atmospheric model fitting we estimate a surface gravityof $\log g$=5 and an effective temperature of ~500-600K. A comparison of itsspectrum with observed T dwarfs indicates a late-T spectral type, with a T9object providing the best match. By combining the observed astrometry from theimaging data with 27 years of radial velocities, we use orbital fitting toconstrain its orbital and physical parameters, as well as update those of theplanet HD4113Ab, discovered by previous radial velocity measurements. The datasuggest a dynamical mass of $66\pm5$M$_J$ and moderate eccentricity of$0.44\pm0.08$ for the brown dwarf. This mass estimate appears to conflict withthe isochronal estimate and that of similar objects, which may be caused by thenewly detected object being an unresolved binary brown dwarf system or thepresence of an additional object in the system. Through dynamical simulationswe show that the planet may undergo strong Lidov-Kozai cycles, raising thepossibility that it formed on a quasi-circular orbit and gained its currentlyobserved high eccentricity through interactions with the brown dwarf. Follow-upobservations combining radial velocities, direct imaging and Gaia astrometrywill be crucial to precisely constrain the dynamical mass of the brown dwarfand allow for in-depth comparison with evolutionary and atmospheric models.

Direct imaging of an ultracool substellar companion to the exoplanet host star HD 4113A