  As part of a radial velocity survey of low Galactic latitude structures that
we undertook with the 2dF spectrograph on the AAT, we present the radial
velocities of more than 1500 Red Giant Branch and Red Clump stars towards the
centre of the Canis Major dwarf galaxy. With a mean velocity of 72\pm7 km/s at
a Heliocentric distance of 5.5 kpc and 114\pm2 km/s at 8.5 kpc, these stars
present a peculiar distance -- radial velocity relation that is unlike that
expected from thin or thick disc stars. Moreover, they belong to a
kinematically cold population with an intrinsic dispersion that may be as low
as 11_{-1}^{+3} km/s. The radial velocity distribution is used to select Canis
Major stars in the UCAC2.0 proper motion catalogue and derive proper motions in
Galactic coordinates of (\mu_l,\mu_b)= (-3.6\pm0.8 mas/yr, 1.5\pm0.4 mas/yr)
for the dwarf galaxy, which after correcting for the reflex solar motion along
this line-of-sight gives (\mu_l',\mu_b')= (-6.8\pm0.8 mas/yr, 0.8\pm0.4
mas/yr)$, corresponding to a prograde orbit with a tangential velocity of
\sim235 km/s at the average distance of \sim7.2 kpc. All these kinematic
constraints can be reproduced in simulations of the accretion of a dwarf onto
the Galactic disc. Such a process could also be responsible for the Monoceros
Ring that has recently been shown to encompass the Galactic disc. However,
without constraints on the kinematics of the tidal arms emerging from the Canis
Major dwarf, it is not yet possible to definitively prove a link between the
two structures.
