Time-resolved X-Shooter spectra and RXTE light curves of the ultra-compact X-ray binary candidate 4U 0614+091

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 analysed previously by Nelemans et al. We confirm the presence of CIII and OII emission features at ≈4650 Å and ≈5000 Å. The emission lines do not show evident Doppler shifts that could be attributed to the motion of the donor star/hotspot around the centre 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 Å. 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 c