The sample of hadronic Z decays collected by the Aleph detector at Lep in the years 1991-1995 is analysed in order to measure the forward-backward asymmetry in Z→bb¯ and Z→cc¯ events and the B0−B¯0 average mixing parameter χ¯ . Quark charges are tagged by the charges of electrons and muons produced in b and c semileptonic decays. Multivariate analyses are used to separate the event flavours and b→ℓ / b→c→ℓ processes. The b and c quark asymmetries are measured simultaneously; the average mixing parameter and the pole asymmetries are determined to be
χ¯=0.1196±0.0049(stat.)+0.0043−0.0050(syst.),A0,bFB=0.0998±0.0040(stat.)±0.0017(syst.),A0,cFB=0.0732±0.0053(stat.)±0.0037(syst.).
These asymmetries, combined with the Aleph measurements of the b asymmetry using inclusive b hadron decays and of the c asymmetry using reconstructed D mesons, correspond to a value of the effective electroweak mixing angle of sin2θeffW=0.23188±0.00046 .