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Measuring the photon energy spectrum in radiative B decays provides essentialhelp for gaining theoretical control over semileptonic B transitions. Thehadronic recoil mass distribution in B -> X_u \ell\nu promises the bestenvironment for determining |V_ub|. The theoretical uncertainties are largestin the domain of low values of the lepton pair mass q^2. Universality relationsallow to describe this domain reliably in terms of the photon spectrum in B ->X_s + \gamma. A method is proposed to incorporate 1/m_b corrections into thisrelation. The low-E_\gamma tail in radiative decays is important in the contextof extracting |V_ub|. We argue that CLEO's recent fit to the spectrumunderestimates the fraction of the photon spectrum below 2 GeV. Potentiallysignificant uncertainties enter in the theoretical evaluation of the integratedend-point lepton spectrum or the B -> X_u \ell\nu width with a too high valueof the lower cut on q^2 in alternative approaches to |V_ub|.Comment: 24 pages, 6 figures, LaTeX. Revised: Complete version. Numerical  predictions are improved and the estimate for the decay fraction revised. The  theoretical expectations for the decay fraction and the spectrum itself are  given on the plot

ON THE EXPECTED PHOTON SPECTRUM IN B → Xs+ γ AND ITS USES

ON THE EXPECTED PHOTON SPECTRUM IN B → X_s+ γ AND ITS USES