Blue Horizontal Branch Stars in the Sagittarius dwarf spheroidal galaxy

We report on the recovery of a Blue Horizontal Branch (BHB) population belonging to the Sagittarius Dwarf Spheroidal Galaxy (Sgr). The sequence is clearly identified in the (V, V-I) Color Magnitude Diagram (CMD) obtained for about 500,000 stars in the region of the globular cluster M~54. The BHB morphology is similar to the analogous sequence in M~54, but it is unambiguously associated with Sgr since {\it(i)} it is detected well outside the main body of the cluster, up to more than 5 tidal radii from the cluster center and {\it(ii)} the BHB stars follow the radial distribution of the other stellar populations of Sgr. This finding finally demonstrates that the Sgr galaxy hosts a significant (of the order of $\sim$10%) old and metal-poor stellar population ([Fe/H]\ltsima -1.3; age \gtsima 10 Gyr), similar to that of its oldest clusters (M~54, Ter~8). We also show that the Sgr BHB sequence found here is the counterpart of the analogous feature observed by Newberg et al. (2002) in the Sgr Stream, in a field more than 80$\degr$ away from the center of the galaxy.