  The computation on the lattice of the bag parameter $B_{B_s}$ associated to
the $B_s - \bar{B_s}$ mixing amplitude in the Standard Model is presented. The
estimation has been made by combining the static limit of HQET and the
Neuberger light quark action which preserves the chiral symmetry on the
lattice. We find $B^{\bar{MS}}{stat}}_{B_s}(m_b)=0.92(3)$.
