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

Simultaneous NICER and NuSTAR Observations of the Ultra-compact X-ray Binary 4U 0614+091