Let f and g be two regular functions on U smooth affine variety. Let M be a regular holonomic D U -module. We are interested in the irregularity of the complex f + (Me g ). More precisely, we relate the irregularity number at c of the systems) with the characteristic cycles of the systems• Let U be a smooth affine variety over C and g : U → C be a regular function on U . We denote by O U the sheaf of regular functions on U and by D U the sheaf of algebraic differential operators on U .Let In this paper, we consider two regular functions f, g : U → C. We are interested in the irregularity of the cohomology modules of the direct image by f of a D U -module, Me g , where M is regular and holonomic.