Synthetic spectra of HCN (left) and H¹³CN (right) obtained with LOC (top rows: L1544, bottom row: HH211). For L1544, the results are splitted between using the Keto-Caselli model (top) and the HDCRT model (centre) as approximation for the physical structure of the core. Towards L1544, the best-fitting abundances and timesteps are 2 × 10⁻⁸ (KC), 7.9e5 yr (STKC), 1 × 10⁻⁸ (HD), 1.3e5 yr (STHD) for HCN; 2 × 10⁻¹⁰ (KC), 6e5 yr (STKC), 3 × 10⁻¹⁰ (HD), 1e5 yr (STHD) for H¹³CN. Towards HH211, the best-fitting abundances and timesteps are 2 × 10⁻⁹ (const), 1e5 yr (2-phase) for HCN; 3 × 10⁻¹¹ (const), 1.3e5 yr (2-phase) for H¹³CN.