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We present a first deep colour-magnitude diagram of the putative centralregion (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 theremnant of a dwarf satellite accreted onto the Milky Way on a near-equatorialorbit. We find a narrow (in apparent magnitude) main-sequence, extending 6magnitudes below the turn-off to our limiting magnitude of B ~ 24.5 mag. Thismain sequence has very high constrast (>3) with respect to the thin/thickdisk/halo background; its narrowness at brighter magnitudes clearly implies thepresence of a distinct and possibly still bound stellar system. We derived theline-of-sight size (r_{1/2}) of this system based on the B-band width of thelower main sequence, obtaining 0.94 +/- 0.18 (random) +/- 0.18 (systematic)kpc. That size matches a model prediction for the main body of the parentgalaxy of the Monoceros tidal stream. The high density contrast and limitedspatial extent in the radial direction are very hard to reconcile with thealternative explanation put forward to explain the Canis Majorstellar-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 +/- 0.1mag. These values place the Canis Major object in the category of dwarf galaxyin the the L_{V}--size and M_V - mu_{V} planes for such objects. However, likethe Sagittarius dwarf, it is an outlier in the[Fe/H] -M_V plane in the sensethat it is too metal rich for its estimated absolute magnitude. This suggeststhat the main mechanism driving its recent and current star formation history(possibly tidal stripping) is different to that of isolated dwarfs.Comment: Accepted version in Astrophysical Journal Part 1. Include a new  figure, some changes in the derive CMa dwarf properties, main conclusions  unchanged. 5 Pages, 3 figures, 2 tables. High resolution version can be  downloaded from:ftp://ftp.iaa.csic.es/pub/ddelgado/canismajor.ps.g

The Closest View of a Dwarf Galaxy: New Evidence on the Nature of the Canis Major Overdensity