  We present the first joint NuSTAR and NICER observations of the ultra-compact
X-ray binary (UCXB) 4U 0614+091. This source shows quasi-periodic flux
variations on the timescale of ~days. We use reflection modeling techniques to
study various components of the accretion system as the flux varies. We find
that the flux of the reflected emission and the thermal components representing
the disk and the compact object trend closely with the overall flux. However,
the flux of the power-law component representing the illuminating X-ray corona
scales in the opposite direction, increasing as the total flux decreases.
During the lowest flux observation, we see evidence of accretion disk
truncation from roughly 6 gravitational radii to 11.5 gravitational radii. This
is potentially analogous to the truncation seen in black hole low-mass X-ray
binaries, which tends to occur during the low/hard state at sufficiently low
Eddington ratios.
