In the presence of mixing between massive neutrino states, the distortion of the electron spectrum in beta decay is, in general, a function of several masses and mixing angles. For 3nu-schemes which describe the solar and atmospheric neutrino data, this distortion can be described by a single effective mass, under certain conditions. In the literature, two different definitions for the effective mass have been suggested. We show that for quasi-degenerate mass schemes (with an overall mass scale in and splitting Deltam(2)) the two definitions coincide Up to (Deltam(2))(2)/m(4) corrections. We consider the impact of different effective masses on the integral energy spectrum. We show that the spectrum with a single mass can be used also to fit the data in the case of 4nu-schemes motivated, in particular, by the LSND results. In this case the accuracy of the mass determination turns out to be better than (10-15). (C) 2003 Elsevier Science B.V. All rights reserved. References: 22
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