Results from a joint experimental and direct numerical simulation (DNS) investigation are presented for the flow over a backward-facing step manipulated by low-amplitude time-periodic (harmonic) blowing/suction excitation through a narrow slot at the edge of the step. For a Reynolds number of Re_h=3000 (based on step height, h, and inflow velocity, U_o) and for laminar inflow, a 33% reduction of the mean recirculation length (in comparison to the non-manipulated reference case) could be obtained with a forcing amplitude of the order of one per cent of U_o. Based on the momentum thickness, θ, of the incoming laminar boundary layer (at the edge of the step), the corresponding optimum Strouhal number is St_θ=f_optθ/U_o=0.012. From the experimental data it can be concluded that, in our flow case, the optimum frequency, f_opt=50 Hz , was the most amplified frequency in the transition-to-turbulence process of the separated laminar shear layer. Detailed comparison of the experimental data with data from the numerical simulation shows that DNS and experimental data agree up to second-order statistics. The joint experimental and numerical investigations exhibit a complementary nature in the sense that, on the one hand, the main advantage of the experiment was the relative ease with which a wide range of forcing parameters could be tested and, on the other hand, DNS could provide spatio-temporal details of the flow which could not be so easily obtained in the experiment.