Observations of nitrogen isotope fractionation in deeply embedded protostars

CONTEXT: N-enrichment in prestellar cores, it is unclear how the signature evolves into the protostellar phases. AIMS: N ratio around three nearby, embedded low- to intermediate-mass protostars. METHODS: C ratio. The assumption of optically thin emission was verified using radiative transfer modeling and hyperfine structure fitting. RESULTS: N ratios with increasing outer envelope temperature. CONCLUSIONS: N ratios from additional tracers are indispensable for testing the models. Spatially resolved observations are needed to distinguish between chemical fractionation and isotope-selective photochemistry.