tokens: method applied to the observed data as described in Section 3 . For each template the process was repeated 100 times and the distribution of recovered metallicities is shown in Fig. B1 . It can be seen that although there is a systematic offset to lower metallicities for all but the lowest metallicity template (for which it is impossible to measure a lower metallicity), the effect is relatively small (≤10 per cent). We therefore conclude that the low metallicities derived for our VANDELS sample are robust against the effect of redshift uncertainties. Figure B1. The distribution of recovered metallicity for each of the five default Starburst99 WM-Basic template spectra ( Z ∗ = 0.001, 0.002, 0.008, 0.014, and 0.040) after mimicking the effect of redshift uncertainties when creating composite spectra (see text for details). Each panel shows one of the five default templates with the true metallicity indicated by the dashed vertical red line. The distribution of recovered metallicities is shown in black. The offset between the median recovered metallicity and the true metallicity is quoted in the top left-hand corner of each panel. APPENDIX C: BPASSv2.1 STELLAR POPULATION SYNTHESIS MODELS In our main analysis, we focused on using the SB99 WM-Basic stellar population synthesis models to fit the VANDELS spectra for the reasons outlined in Section 3 . However, we have also performed the same analysis using the BPASSv2.1 stellar population synthesis models described in Eldridge et al. ( 2017 ). It is well known that using different models will cause systematic offsets in derived parameters (e.g. Conroy 2013 ; Cid Fernandes et al. 2014 ) and it is useful to have an estimate of how significant this offset might be in the case of FUV-derived metallicities. The unique feature of the BPASSv2.1 models is the inclusion of massive binary star evolution, which can have a strong effect on the predicted UV spectrum, particularly at low metallicities. For example, at Z ∗ /Z ⊙ 0.35, a phenomenon known as ‘quasi-homogeneous evolution’ can occur (QHE; e.g. Eldridge Stanway 2012 ), which results in stars living longer on the main sequence and becoming hotter than single stars at similar mass and metallicity. QHE can have a profound effect on the estimated ages and ionizing photon output of stellar populations. Stellar metallicities should be less affected, particularly since the O-star models in BPASSv2.1 are also generated with WM-Basic code, albeit with a different parameter set to SB99. Nevertheless, it worth testing this assumption. We considered a 100 Myr constant star formation BPASSv2.1 model that included binary star evolution and had an IMF cut-off of 100 M ⊙ , with IMF index of −1.3 between 0.1–0.5 M ⊙ and −2.35 above 0.5 M ⊙ . The model is referred to as BPASSv2.1-100bin. We considered the following set of metallicity values Z ∗ = (0.001, 0.002, 0.003, 0.004, 0.006, 0.008, 0.010, 0.014, 0.020, 0.030, 0.040), interpolating between the models in the same
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