NaI doublet in Doppler-corrected spectra of B312254, aka NV374 (B − V = 0.16), and B311900 (B − V = 0.64). The observed spectra (solid lines) are compared to synthetic templates assigned as explained in Sect. 3.2 (broken lines). The templates are shifted upwards by 0.02 for clarity. Since the stellar spectra have been Doppler-corrected to the velocity obtained from H_β, the ISM components appear shifted with respect to the rest wavelengths.