a) Comparison between 1-g best-fit RVs from Hα and [N II] 6584 lines. Black dots indicate pure-sky fibres while gray dots indicate faint stars. The dashed line indicates identity. b) Same comparison as in panel a, between Hα and [S II] 6731. c) Same comparison as in panel a, between [N II] 6584 and [S II] 6731. d) Comparison between intensities of Hα and [N II] 6584, as resulting from 1-g models. Symbols as in panel a. The dashed line represents a ratio of 1/8.