Measurements of $$B^0 - \bar B^0 $$ mixing, $${{\Gamma (Z^0 \to b\bar b)} \mathord{\left/ {\vphantom {{\Gamma (Z^0 \to b\bar b)} {\Gamma (Z^0 \to hadrons)}}} \right. \kern-\nulldelimiterspace} {\Gamma (Z^0 \to hadrons)}}$$ and semileptonic branching ratios forb-flavoured hadrons in hadronicZ 0 decays