Nitrogen isotopic ratios in N₂H⁺ obtained in this work (red stars) compared with the values obtained towards prestellar cores (blue dots, Redaelli et al. 2018). L1544 and L694-2 present two values because in those sources both N¹⁵NH⁺ and ¹⁵NNH⁺ were detected. The blue dashed curve shows the weighted average of the prestellar sample, and the shaded area the associated uncertainty. The red dot-dashed curve is the average for the protostellar objects. The black solid line shows the elemental value of ¹⁴N/¹⁵N = 440.