  We discuss the role that dwarf galaxies may have played in the formation of
the Galactic halo (Halo) using RR Lyrae stars (RRL) as tracers of their ancient
stellar component. The comparison is performed using two observables (periods,
luminosity amplitudes) that are reddening and distance independent.
  Fundamental mode RRL in six dwarf spheroidals and eleven ultra faint dwarf
galaxies (1,300) show a Gaussian period distribution well peaked around a mean
period of <Pab>=0.610+-0.001 days (sigma=0.03). The Halo RRL (15,000) are
characterized by a broader period distribution.
  The fundamental mode RRL in all the dwarf spheroidals apart from Sagittarius
are completely lacking in High Amplitude Short Period (HASP) variables, defined
as those having P< 0.48 days and Av> 0.75mag. Such variables are not uncommon
in the Halo and among the globular clusters and massive dwarf irregulars. To
further interpret this evidence, we considered eighteen globulars covering a
broad range in metallicity (-2.3< [Fe/H]< -1.1) and hosting more than 35 RRL
each.
  The metallicity turns out to be the main parameter, since only globulars more
metal--rich than [Fe/H] -1.5 host RRL in the HASP region. This finding suggests
that dSphs similar to the surviving ones do not appear to be the major
building-blocks of the Halo. Leading physical arguments suggest an extreme
upper limit of 50% to their contribution. On the other hand, massive dwarfs
hosting an old population with a broad metallicity distribution (Large
Magellanic Cloud, Sagittarius) may have played a primary role in the formation
of the Halo.

Solar and Stellar Astrophysics;Astrophysics of Galaxies