The recently proposed theory of vibronic transitions in impurity centres of solids [10] is applied to zero-phonon lines in optical spectra. New equations for the relative intensity, position and width of the line are derived, which take into account both the linear and the quadratic electron-phonon couplings. The joint effect of these couplings on the line intensity is found to be different for absorption and luminescence. (C) 2010 Published by Elsevier B. V.