  A study of p(gamma,K)Sigma processes in an isobar model at tree level is
reported. By comparing model calculations to the published SAPHIR data, we
explore the possible role of different isospin I=1/2 (N*) and I=3/2 (Delta*)
resonances in the reaction dynamics. In our analysis, the inclusion of the
``missing'' D_{13}(1895) resonance does only slightly improve the global
description of the Sigma photoproduction data. More convincing signals for the
presence of such a ``missing'' resonance emerged in the analysis of the isospin
related p(gamma,K+)Lambda reaction. Various implementations of the nonresonant
part of the Sigma photoproduction amplitude are presented. The sensitivity of
the computed observables and extracted resonance parameters to the
uncertainties inherent to the treatment of the nonresonant (background)
diagrams are discussed.
