Ds+ mesons produced inZ0→bb events were separated from theZ0→cc component using a lifetime tag. Using a sample of 1.5 million hadronicZ decays collected with theAleph detector theBs0 andDs+ yields have been measured:B(b→B ¯ 0 s )B(B ¯ 0 s →D + s )=0.088±0.020(stat.) ±0.020(stat.) B(c→D + s )=0.088±0.020(stat.) +0.019 −0.016 (syst.) TheBs0 lifetime was measured in aBs0 enriched sample reconstructing the decay length from the vertex of theDs+ with a hadron from theBs0 decay. The result obtained is:τ B s =1.61 +0.30 −0.29 (stat.) +0.018 −0.016 (syst.) ps.