  Standard cosmology predicts that dwarfs were the first galaxies to be formed
in the Universe and that many of them merge afterwards to form bigger galaxies
such as the Milky Way. This process would have left behind traces such as tidal
debris or star streams in the outer halo. We report here the detection of a
very low density stellar system at 50+/- 10 kpc from the Galactic center that
could be related to the merger process. It could form part of the Sagittarius
northern stream or, alternatively, could be the trace of a hitherto unknown
dwarf galaxy.
  The dwarf galaxy in Sagittarius, the closest satellite of the Milky Way, is
currently being tidally disrupted and is a ``living'' test for galaxy formation
theories. The system found here is 60 deg away from the center of the
Sagittarius galaxy. If it is really associated with this galaxy, it would
confirm predictions of dynamical interaction models indicating that tidal
debris from Sagittarius could extend along a stream completely enveloping the
Milky Way in a polar orbit.
