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tokens: [α/Fe] = +0.4 Dartmouth isochrone. The dashed outline is the final adopted selection window as discussed in Section 2.2 . The agreement between the observations and the theoretical isochrone is good: for 0.6 ≤ ( g − i ) 0 ≤ 0.8 where the isochrone is approximately constant at m i = 0.029 mag, the mean of the globular cluster stars is m i = 0.033 with a standard deviation σ = 0.040 mag. The observed scatter in the globular cluster m i values is large but there is no obvious indication of a systematic offset between the observations and the theoretical isochrone. The scatter is presumably driven by uncertainties in the v magnitudes as the corresponding i 0 , ( g − i ) 0 combined colour–magnitude diagram (generated using an appropriate relative distance modulus offset) shows only a small scatter in ( g − i ) 0 : σ( g − i ) 0 = 0.021 mag about the mean RGB locus. 2.1.2 Known EMP stars An alternative approach is to investigate the location of known EMP stars in the metallicity-sensitive diagram. To perform this task we have compiled from the literature a list of stars with [Fe/H] ≤ −2.5, as determined from high-dispersion spectroscopy. The list was then cross-matched with the SkyMapper DR1.1 data base. In carrying out the match-up we have used tighter limits on the SkyMapper photometry in order to reduce the effects of photometric uncertainty on the location of the stars in the metallicity-sensitive diagram. Specifically, we required e_g_psf AND e_i_psf ≤ 0.02, e_v_psf ≤ 0.03, and E( B − V ) SFD 0.1 mag, and this resulted in a sample of 294 stars. The majority of the stars are drawn from Yong et al. ( 2013 ), Jacobson et al. ( 2015 ), Barklem et al. ( 2005 ), Roederer et al. ( 2014 ), and Cohen et al. ( 2013 ); the effective temperatures, gravities, and metallicities from the latter two sources have been adjusted to ensure consistency with the other sources. The location of the known abundance stars in the metallicity-sensitive diagram is shown in Fig. 3 . We note that the most iron-poor star known, star SMSS J031300.36−670839.3, is the dark coloured five-point star symbol at (( g − i ) 0 , m i ) = (0.62, −0.09) in the figure, while the star HE 0107−5240, which has [Fe/H] ≈ −5.4, is the dark coloured five-point star symbol at (0.59, −0.135). The subgiant star HE 1327−2326 ([Fe/H] ≈ −5.7) is located at (0.29, 0.19), and is also plotted as a dark coloured five-point star symbol in Fig. 3 . In general, the majority of the stars plotted are consistent with expectations in that they lie at notably bluer m i values at a given ( g − i ) 0 than the bulk of the population. However, this is clearly not the case for a small number of known EMP stars that fall well below their ‘expected’ location in the metallicity-sensitive diagram. As will be discussed in more detail in Section 4.1 , generally these stars are strongly enhanced in carbon. Figure 3. SkyMapper metallicity-sensitive diagram for all objects satisfying the basic selection criteria. Overplotted