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We obtained deep images of G 196-3 B and VHS J1256-1257 b with the NOrthern Extended Millimeter Array (NOEMA) at 1.3 mm. These data were combined with recently published Atacama Large Millimeter Array (ALMA) and Very Large Array (VLA) data of VHS J1256-1257 b at 0.87 mm and 0.9 cm, respectively. Neither G 196-3 B nor VHS J1256-1257 b were detected in the NOEMA, ALMA and VLA data. At 1.3 mm, we imposed flux upper limits of 0.108 mJy (G 196-3 B) and 0.153 mJy (VHS J1256-1257 b) with a 3-sigma confidence. Using the flux upper limits at the millimeter and radio wavelength regimes, we derived maximum values of 0.016 M$_{\rm Earth}$ and 0.004 M$_{\rm Earth}$ for the mass of any cold dust that might be surrounding G 196-3 B and VHS J1256-1257 b, respectively. We put our results in the context of other deep millimeter observations of free-floating and companion objects with substellar masses smaller than 20 M$_{\rm Jupiter}$ and ages between 1 and a few hundred million years. Only two very young objects are detected out of a few tens concluding, as other groups did before, that the disks around these very low-mass objects must have small masses and possibly reduced sizes. If debris disks around substellar objects scale down in a similar manner as protoplanetary disks do, millimeter observations of moderately young brown dwarfs and planets must be at least two orders of magnitude deeper for being able to detect and characterize their surrounding debris disks., Comment: Accepted fro publication in A&A

Radio observations of ultracool dwarfs, objects comprising brown dwarfs and the very lowest mass stars, have mainly focused on analyzing their light-curve and spectral energy distributions providing valuable insights into their magnetic fields. However, spatially-resolved studies of such magnetospheres have been elusive so far. Radio interferometric observations of the brown dwarf LSR J1835+3259 reveal an extended magnetosphere with a morphology compatible with the presence of a radiation belt, similar to that of Jupiter and Earth, consisting of energetic particles confined via magnetic mirroring. Our finding suggests that radio emitting ultracool dwarfs may behave as scaled up versions of Jupiter, validating the connection between dipole-ordered magnetic fields and the presence of belt-like morphologies and aurorae beyond our Solar System., Comment: Submitted on 12 January 2023 to Science. In review