  In this paper we present X-Shooter time resolved spectroscopy and RXTE PCA
light curves of the ultra-compact X-ray binary candidate 4U 0614+091. The
X-Shooter data are compared to the GMOS data analyzed previously by Nelemans et
al. (2004). We confirm the presence of C III and O II emission features at ~
4650 {\AA} and ~ 5000 {\AA}. The emission lines do not show evident Doppler
shifts that could be attributed to the motion of the donor star/hot spot around
the center of mass of the binary. We note a weak periodic signal in the
red-wing/blue-wing flux ratio of the emission feature at ~ 4650 {\AA}. The
signal occurs at P = 30.23 +/- 0.03 min in the X-Shooter and at P = 30.468 +/-
0.006 min in the GMOS spectra when the source was in the low/hard state. Due to
aliasing effects the period in the GMOS and X-Shooter data could well be the
same. We deem it likely that the orbital period is thus close to 30 min,
however, as several photometric periods have been reported for this source in
the literature already, further confirmation of the 30 min period is warranted.
We compare the surface area of the donor star and the disc of 4U 0614+091 with
the surface area of the donor star and the disc in typical hydrogen-rich
low-mass X-ray binaries and the class of AM Canum Venaticorum stars and argue
that the optical emission in 4U 0614+091 is likely dominated by the disc
emission. Additionally, we search for periodic signals in all the publicly
available RXTE PCA light curves of 4U 0614+091 which could be associated with
the orbital period of this source. A modulation at the orbital period with an
amplitude of ~ 10% such as those that have been found in other ultra-compact
X-ray binaries (4U 0513-40, 4U 1820-30) is not present in 4U 0614+091.

High Energy Astrophysical Phenomena