We acknowledge in particular the support of Austrian Federal Ministry of Science and Traffics, GZ 616.364/2-III/2a/98, FNRS–FWO, Belgium, FINEP, CNPq, CAPES, FUJB and FAPERJ, Brazil, Czech Ministry of Industry and Trade, GA CR 202/96/0450 and GA AVCR A1010521, Danish Natural Research Council, Commission of the European Communities (DG XII), Direction des Sciences de la Matire, CEA, France, Bundesministerium fur Bildung, Wissenschaft, Forschung und Technologie, Germany, General Secretariat for Research and Technology, Greece, National Science Foundation (NWO) and Foundation for Research on Matter (FOM), The Netherlands, Norwegian Research Council, State Committee for Scientific Research, Poland, 2P03B06015, 2P03B1116 and SPUB/P03/178/98, JNICT–Junta Nacional de Investigação Cientı́fica e Tecnologica, Portugal, Vedecka grantova agentura MS SR, Slovakia, Nr. 95/5195/134, Ministry of Science and Technology of the Republic of Slovenia, CICYT, Spain, AEN96–1661 and AEN96-1681, The Swedish Natural Science Research Council, Particle Physics and Astronomy Research Council, UK, Department of Energy, USA, DE–FG02–94ER40817.

The ratio of the CKM quark-mixing matrix elements |Vub|/|Vcb| has been measured using B hadron semileptonic decays. The analysis uses the reconstructed mass MX of the secondary hadronic system produced in association with an identified lepton. Since BªX lln transitions are characterised by hadronic masses below those of the D mesons produced in BªX lln transitions, events with a reconstructed value of MX significantly below the D mass are selected. Further signal enrichments are obtained using the topology of reconstructed decays and hadron identification. A fit to the numbers of decays in the b→u enriched and depleted samples with MX above and below 1.6 GeV/c2 and to the shapes of the lepton energy distribution in the B rest frame gives V rV s0.103 stat."0.016 syst."0.010 model and, correspondingly, a charmless semileptonic B decay branching fraction of BR(BRBªX llns1.57"0.35 stat."0.48 syst." u .. y3 0.27model =10.

DELPHI Collaboration: et al.

Peer reviewed