Flavour structure of supersymmetric SO(10) GUTs with extended matter sector

We discuss in detail the flavour structure of the supersymmetric SO(10) grand unified models with the three traditional 16-dimensional matter spinors mixed with a set of extra 10-dimensional vector multiplets which can provide the desired sensitivity of the SM matter spectrum to the GUT symmetry breakdown at the renormalizable level. We put the qualitative argument that a successful fit of the quark and lepton data requires an active participation of more than a single vector matter multiplet on a firm, quantitative ground. We find that the strict no-go obtained for the fits of the charged sector observables in case of a single active matter 10 is relaxed if a second vector multiplet is added to the matter sector and excellent, though non-trivial, fits can be devised. Exploiting the unique calculable part of the neutrino mass matrix governed by the SU(2)_L triplet in the 54-dimensional Higgs multiplet, a pair of genuine predictions of the current setting is identified: a non-zero value of the leptonic 1-3 mixing close to the current 90% C.L. limit and a small leptonic Dirac CP phase are strongly preferred by all solutions with the global-fit chi^2-values below 50.