CCD photometry of the Tucana dwarf galaxy.

We present V and I CCD photometry for ~360 stars in the recently discovered dwarf galaxy Tucana. The large field investigated and the accurate photometric calibration make our data complementary to the deeper HST photometry. From the I magnitude of the tip of the red giant branch we estimate a distance modulus (m-M)_0_=24.69+/-0.16, corresponding to 870+/-60Kpc, confirming that Tucana is an isolated dwarf spheroidal located almost at the border of the Local Group. From the color of the red giant branch tip and by direct comparison with the giant branches of galactic globular clusters we estimate a metallicity [Fe/H]=-1.8+/-0.2, with no clear indication for a metallicity spread. The color-magnitude diagram indicates that Tucana has had a single star formation burst at the epoch of the Galactic globular cluster star formation. There is no evidence for an intermediate or young stellar population. We derive the V luminosity profile, the surface density profile of resolved stars, and the structural parameters of Tucana, from which we confirm that Tucana participates to the general metallicity-surface brightness-absolute magnitude relations defined by the Galaxy and M31 dwarf spheroidal and dwarf elliptical companions.