  We present a first deep colour-magnitude diagram of the putative central
region (0.5 deg x 0.5 deg) of the Canis Major stellar over-density
(l,b)=(240,-8) found by Martin et al. (2004), which has been proposed as the
remnant of a dwarf satellite accreted onto the Milky Way on a near-equatorial
orbit. We find a narrow (in apparent magnitude) main-sequence, extending 6
magnitudes below the turn-off to our limiting magnitude of B ~ 24.5 mag. This
main sequence has very high constrast (>3) with respect to the thin/thick
disk/halo background; its narrowness at brighter magnitudes clearly implies the
presence of a distinct and possibly still bound stellar system. We derived the
line-of-sight size (r_{1/2}) of this system based on the B-band width of the
lower main sequence, obtaining 0.94 +/- 0.18 (random) +/- 0.18 (systematic)
kpc. That size matches a model prediction for the main body of the parent
galaxy of the Monoceros tidal stream. The high density contrast and limited
spatial extent in the radial direction are very hard to reconcile with the
alternative explanation put forward to explain the Canis Major
stellar-overdensity: a flared or warped Galactic disk viewed in projection
(Momamy et al. 2004). We also derived a central surface brightness of mu_{V,0}=
23.3 +/- 0.1 mag arcsec^{-2} and an absolute magnitude of M_{V}=-14.5 +/- 0.1
mag. These values place the Canis Major object in the category of dwarf galaxy
in the the L_{V}--size and M_V - mu_{V} planes for such objects. However, like
the Sagittarius dwarf, it is an outlier in the[Fe/H] -M_V plane in the sense
that it is too metal rich for its estimated absolute magnitude. This suggests
that the main mechanism driving its recent and current star formation history
(possibly tidal stripping) is different to that of isolated dwarfs.
