Standard cosmology predicts that dwarfs were the first galaxies to be formed in the universe and that many of them merge afterward 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° 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.