R-matrix theory is a general framework for describing nuclear reactions (neutron- and charged-particle-induced) that is particularly well-suited for including resonances. We will review some unconventional properties of resonances that arise from this theory, including non-Breit-Wigner (BW) resonances and shadow poles, and discuss their physical consequences. Examples will be given from the analyses of several light systems that have been used in ENDF evaluations, including (sup 4)He, (sup 5)He, (sup 15)N, and (sup 17)O. The resonances in the helium systems tend to be significantly non-BW in character, while almost all the resonances in (sup 15)N and (sup 17)O are Breit-Wigner. An interesting exception in the case of (sup 15)N indicates that some of the sub-threshold levels that have been assumed to be bound might be virtual. We find that fitting data from all possible reactions simultaneously results in level schemes for the compound systems that differ in some cases significantly from the ''accepted'' published level information.
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