Novel relations between lepton and quark mixings

A novel relation is found which gives the 2-3 lepton mixing angle theta(mu tau) in terms of quark; masses and CKM mixing: tan theta(mu tau) = (m(b)/m(s))V-cb. This relation, which is remarkably in good agreement with the quark data and the large lepton mixing recently observed, is a kind of SO(10) GUT relation similar to the celebrated bottom-tau mass ratio in SU(5). The GUT models in which this relation hold should have a 'twisted SO(10)' structure between the second and third generations, in order to really explain the large lepton mixing, or equivalently, the light-hand side of the relation. (C) 2000 Elsevier Science B.V. All rights reserved.