We report the discovery of a 1mHz optical quasi-periodic oscillation (QPO) in
the candidate ultracompact low-mass X-ray binary 4U 0614+091. The ultra-low
frequency QPO has no X-ray counterpart in contemporaneous RXTE/PCA data and is
likely a signature of structure in the accretion disk. The QPO can be
reasonably fitted with a single sine wave but with a phase jump part way
through the observation, indicating that it is not coherent.We also identify a
48 min modulation, approximately consistent with the suggested orbital period
of O'Brien (2005) and Shahbaz et al. (2008). If this is indeed orbital, it
supports an identification of 4U 0614+091 as an ultra-compact source.Comment: 7 pages, 5 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Societ
