Isotopic fractionation can serve as a powerful tracer of chemical evolution during star and planet formation. To accurately interpret observations, it is crucial to identify the dominant pathways of nitrogen and carbon fractionation at different evolutionary stages. We aim to study nitrogen and carbon fractionation in a sample of young cores at the onset of star formation. We mapped H 13 CN and HC^15N around one starless and three pre-stellar cores. We computed the N^ Herschel 15 N maps using the double isotope method for comparison with earlier studies. We compared these results with astrochemical modeling of carbon and nitrogen fractionation for a 1D pre-stellar core model. The computed N(H13CN)/N_2) in three out of four cores. Our analysis reveals a correlation between the H 13 CN/HC^15N ratios and the N^ 13 C. Furthermore, the leading cause of the isotopic variation in the model is not isotope-selective photodissociation, but rather more efficient fractionation through exchange reactions at lower temperatures in the denser regions of the cores.