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Brown dwarfs in close-in orbits around white dwarfs offer an excellentopportunity to investigate properties of fast-rotating, tidally-locked, andhighly-irradiated atmospheres. We present Hubble Space Telescope Wide FieldCamera 3 G141 phase-resolved observations of two brown dwarf-white dwarfbinaries: WD 0137-349 and EPIC 212235321. Their 1.1 to 1.7 $\mu$m phase curvesdemonstrate rotational modulations with semi-amplitudes of $5.27\pm0.02$% and$29.1\pm0.1$%; both can be well fit by multi-order Fourier series models. Thehigh-order Fourier components have the same phase as the first order and arelikely caused by hot spots located at the substellar points, suggestinginefficient day/night heat transfer. Both brown dwarfs' phase-resolved spectracan be accurately represented by linear combinations of their day- andnight-side spectra. Fitting the irradiated brown dwarf model grids to theday-side spectra require a filling factor of ~50%, further supporting a hotspot dominating the emission of the day-sides. The night-side spectrum of WD0137-349B is reasonably well fit by non-irradiated substellar models and theone of EPIC 212235321B can be approximated by a Planck function. We find strongspectral variations in the brown dwarfs' day/night flux and brightnesstemperature contrasts, which highlights the limitations of band-integratedmeasurements in probing heat transfer in irradiated objects. On thecolor-magnitude diagram, WD 0137-349B evolves along a cloudless model trackconnecting the early-L and mid-T spectral types, demonstrating that clouds anddisequilibrium chemistry have a negligible effect on this object. A fullinterpretation of these high-quality phase-resolved spectra calls for newmodels that couple atmospheric circulation and radiative transfer underhigh-irradiation conditions.

HST/WFC3 Complete Phase-resolved Spectroscopy of White Dwarf-Brown Dwarf Binaries WD 0137 and EPIC 2122