Singularities in general relativity coupled to nonlinear electrodynamics

We study here some consequences of the nonlinearities of the electromagneti c field acting as a source of Einstein's equations on the propagation of ph otons. We restrict to the particular case of a 'regular black hole', and sh ow that there exist singularities in the effective geometry. These singular ities may be hidden behind a horizon or be naked, according to the value of a parameter. Some unusual properties of this solution are also analysed.