  We aim to determine the $^{14}$N/$^{15}$N and $^{12}$C/$^{13}$C ratios for
HCN in six starless and prestellar cores and compare the results between the
direct method using radiative transfer modeling and the indirect double isotope
method assuming a fixed $^{12}$C/$^{13}$C ratio. We present IRAM 30m
observations of the HCN 1-0, HCN 3-2, HC15N 1-0 and H13CN 1-0 transitions
toward six embedded cores. The ${}^{14}$N/${}^{15}$N ratio was derived using
both the indirect double isotope method and directly through non-local
thermodynamic equilibrium (NLTE) 1D radiative transfer modeling of the HCN
emission. The latter also provides the ${}^{12}$C/${}^{13}$C ratio, which we
compared to the local interstellar value. The derived ${}^{14}$N/${}^{15}$N
ratios using the indirect method are generally in the range of 300-550. This
result could suggest an evolutionary trend in the nitrogen fractionation of HCN
between starless cores and later stages of the star formation process. However,
the direct method reveals lower fractionation ratios of around $\sim$250,
mainly resulting from a lower ${}^{12}$C/${}^{13}$C ratio in the range
$\sim$20-40, as compared to the local interstellar medium value of 68. This
study reveals a significant difference between the nitrogen fractionation ratio
in HCN derived using direct and indirect methods. This can influence the
interpretation of the chemical evolution and reveal the pitfalls of the
indirect double isotope method for fractionation studies. However, the direct
method is challenging, as it requires well-constrained source models to produce
accurate results. No trend in the nitrogen fractionation of HCN between earlier
and later stages of the star formation process is evident when the results of
the direct method are considered.
