A comparison between RAVE DR5 and Gaia DR2 Radial Velocities
  This research note presents a comparison of radial velocities between the
RAVE DR5 data release with Gaia DR2. RAVE DR5 and Gaia DR2 have 450587 stars in
common. In particular we would like to shed light on a small subset of joint
targets (707 stars) that exhibit a constant velocity offset of +105 km/s or -76
km/s, respectively. Overall this comparison confirms the excellent agreement
between those two data sets. The velocity differences can well be matched with
two Gaussians with FWHM of 1.2 km/s and 3.6 km/s, respectively. There is a
systematic offset of about -0.32 km/s, consistent with the findings in Gaia
DR2, who find an offset of -0.29 km/s. The offset is also comparable to the
offset found between Gaia DR2 and other ground-based spectroscopic surveys in a
similar magnitude range, indicative that the source for this offset may at
least partially be related to the radial velocity zeropoint of Gaia DR2. We
identified 707 stars that exhibit large constant velocity offsets. These cases
can be traced to a problem of the reduction pipeline in propagating the
wavelength calibration between neighboring fibres near the edges of the field
plate, in particular for observations done just before fibre repairs, i.e. when
live fibres were separated by large gaps of broken ones. In 82\%\ of these
cases the offset between RAVE DR5 and Gaia DR2 radial velocity is in the $+105
\pm 15$~\kms range. These spectra belong to the first 10 fibres of the field
plate. Another 3\%\ of the spectra have velocity offset in the $-76 \pm 4$~\kms
range and were observed with the fibres 148-150. Even though the majority of
these 707 stars do not exhibit any particularity in the radial velocity, we
recommend excluding them from future studies.
