X-SHYNE: X-shooter spectra of young exoplanet analogs. I. A medium-resolution 0.65-2.5$\mathrm{\mu m}$ one-shot spectrum of VHS\,1256-1257 b

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We present simultaneous 0.65-2.5 microns medium resolution (3300 < R < 8100) VLT/X-Shooter spectra of the young low-mass (19+/-5MJup) L-T transition object VHS 1256-1257 b, a known spectroscopic analogue of HR8799d.

• The companion is a prime target for the JWST Early Release Science (ERS) and one of the highest-amplitude variable brown-dwarf known to date.
• We compare the spectrum to the custom grids of cloudless ATMO models exploring different atmospheric composition with the Bayesian inference tool ForMoSA.
• We also re-analyze low-resolution HST/WFC3 1.10-1.67 microns spectra at minimum and maximum variability to contextualize the X-Shooter data interpretation.