Optical spectra for the subsample of spectroscopically characterised sources. Each spectrum, except for the Mookodi spectrum, is de-reddened for visualisation purposes using the best extinction values (see Sect. 6.1, Table B.1), with Fitzpatrick (1999) dust extinction function and R_V = 3.1. The main emission and absorption lines are indicated. The areas with strong telluric absorption are shown by the shaded rectangles. On the right, the number in parentheses indicates the index associated with each source (see Table B.2), and this index is followed by the instrument used to take each spectrum: CAFOS (CAF) from the Calar Alto Observatory 2.2 m telescope, HERMES from the Mercator telescope, FIES and ALFOSC (ALF) from the Nordic Optical Telescope, and Mookodi from the South African Astronomical Observatory 1 m Lesedi telescope.