  The re-emergence of the 0.99 $\mu$m FeH feature in brown dwarfs of early- to
mid-T spectral type has been suggested as evidence for cloud disruption where
flux from deep, hot regions below the Fe cloud deck can emerge. The same
mechanism could account for color changes at the L/T transition and photometric
variability. We present the first observations of spectroscopic variability of
brown dwarfs covering the 0.99 $\mu$m FeH feature. We observed the spatially
resolved very nearby brown dwarf binary WISE J104915.57-531906.1 (Luhman 16AB),
a late-L and early-T dwarf, with HST/WFC3 in the G102 grism at 0.8-1.15 $\mu$m.
We find significant variability at all wavelengths for both brown dwarfs, with
peak-to-valley amplitudes of 9.3% for Luhman 16B and 4.5% for Luhman 16A. This
represents the first unambiguous detection of variability in Luhman 16A. We
estimate a rotational period between 4.5 and 5.5 h, very similar to Luhman 16B.
Variability in both components complicates the interpretation of spatially
unresolved observations. The probability for finding large amplitude
variability in any two brown dwarfs is less than 10%. Our finding may suggest
that a common but yet unknown feature of the binary is important for the
occurrence of variability. For both objects, the amplitude is nearly constant
at all wavelengths except in the deep K I feature below 0.84 $\mu$m. No
variations are seen across the 0.99 $\mu$m FeH feature. The observations lend
strong further support to cloud height variations rather than holes in the
silicate clouds, but cannot fully rule out holes in the iron clouds. We
re-evaluate the diagnostic potential of the FeH feature as a tracer of cloud
patchiness.
