Synthetic spectra of HCO⁺ and isotopologues 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 7 × 10⁻¹⁰ (KC), 9.8e5 yr (STKC), 3 × 10⁻⁹ (HD), 9.8e5 yr (STHD) for HCO⁺; 2 × 10⁻¹⁰ (KC), 1e5 yr (STKC), 4 × 10⁻¹⁰ (HD), 1e5 yr (STHD) for H¹³CO⁺; 6 × 10⁻¹² (KC), 1e5 yr (STKC), 1 × 10⁻¹¹ (HD), 1e5 yr (STHD) for HC¹⁸O⁺ ; 2 × 10⁻¹⁰ (KC), 1.7e5 yr (STKC), 4 × 10⁻¹⁰ (HD), 1.7e5 yr (STHD) for DCO⁺. Towards HH211, the best-fitting abundances and timesteps are 1 × 10⁻¹¹ (const), 1e6 yr (2-phase) for HCO⁺; 7× 10⁻¹² (const), 1e5 yr (2-phase) for H¹³CO⁺; 4× 10⁻¹³ (const), 1e5 yr (2-phase) for HC¹⁸O⁺; 2× 10⁻¹¹ (const), 1e6 yr (2-phase) for DCO⁺.