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tokens: model assumes a temperature of 10 K for the gas, whichis up to 40% higher than the values found for the central parts of the cores (6–7 K). However, further calculations have shown that lowering the temperature by 2–3 K does not produce significant differences in the results (Roueff, priv. comm.). Visser et al. (2018) highlighted how isotope-selective photodissociation is a key mechanism to understand the nitrogen isotopic chemistry in protoplanetary disks. We can speculate that different levels of selective photodissociation in different environments could reproduce the variety of N-isotopic ratios that are observed. Further investigation of this point from both an observational and a theoretical point of view is merited. From the L1544 and L694-2 results, there seems to be tentative evidence that N 15 NH + is more abundant than 15 NNH + . This can be explained by the theory according to which the proton transfer reaction, tends to shift the relative abundance of the two isotopologues, slightly favouring N 15 NH + due to the fact that N 15 NH + zero-point energy is lower than that of 15 NNH + by 8.1 K (see reaction RF2 in Wirström Charnley 2018 ). It is interesting that the same trend is found also in a very different environment such as OMC-2 FIR4, a young protocluster hosting several protostellar objects. In this source, Kahane et al. (2018) measured lower values for the isotopic ratio, but in agreement with our result found that 15 NNH + is less abundant than N 15 NH + . However, we emphasise that higher-quality observations and those with better statistics are needed to confirm this point. 6. Conclusions We have analysed the diazenylium isotopologues’ spectra in three prestellar cores, L183, L429 and L694-2 in order to derive nitrogen isotopic ratios. Since LTE conditions are not fulfilled, especially for the N 2 H + (1–0) transition, we have used a fully non-LTE radiative transfer approach, implemented in the numerical code MOLLIE. We have carefully derived the physical models of the sources, computing their volume density and dust temperature profiles. With these, we were able to produce synthetic spectra to be compared with our observations, in order to derive the best-fit values for the molecular abundances and column densities. Using the same method, we have also recomputed the isotopic ratio of L1544. The difference with the literature value of Bizzocchi et al. (2013) , due to changes in the molecular collisional rates, is well within the uncertainties. In our sample of four cores, we derived 14 N/ 15 N values in the range 580–1000. Within the confidence range given by our uncertainties estimation, all our results are inconsistent with the value ≈400, predicted by the current theoretical models. L1544 still presents higher depletion levels than the other sources, but in general all the cores are anti-fractionated. The theoretical bases of such a trend are at the moment not understood. A deep revision of our knowledge of the