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Rotationally-modulated variability of brown dwarfs and giant planets providesunique information about their surface brightness inhomogeneities, atmosphericcirculation, cloud evolution, vertical atmospheric structure, and rotationalangular momentum. We report results from Hubble Space Telescope/Wide FieldCamera 3 near-infrared time-series spectroscopic observations of threecompanions with masses in or near the planetary regime: VHS J125601.92-125723.9b, GSC 6214-210 B, and ROXs 42 B b. VHS J1256-1257 b exhibits strong totalintensity and spectral variability with a brightness difference of 19.3%between 1.1-1.7 $\mu$m over 8.5 hours and even higher variability at the 24.7%level at 1.27 $\mu$m. The light curve of VHS J1256-1257 b continues to rise atthe end of the observing sequence so these values represent lower limits on thefull variability amplitude at this epoch. This observed variability rivals (andmay surpass) the most variable brown dwarf currently known, 2MASSJ21392676+0220226. The implied rotation period of VHS J1256-1257 b is$\approx$21-24 hr assuming sinusoidal modulations, which is unusually long forsubstellar objects. No significant variability is evident in the light curvesof GSC 6214-210 B ($<$1.2%) and ROXs 42 B b ($<$15.6%). With a spectral type ofL7, an especially red spectrum, and a young age, VHS J1256-1257 b reinforcesemerging patterns between high variability amplitude, low surface gravity, andevolutionary phase near the L/T transition.

Strong Near-Infrared Spectral Variability of the Young Cloudy L Dwarf Companion VHS J1256-1257 b