we discuss our results and the peculiarities of some of the spectra.

Sample selection {#MplusTeff_sdM:sample}
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To select our sample of subdwarfs, we used the SDSS spectroscopic database which contains a wealth of high-quality spectra covering the 5000–9200Å range at a spectral resolution of $\sim$2000. Hundreds of objects have been classified as sdM, esdM, or usdM following the scheme developed by @lepine07c. This classification is publicly available through the SDSS archive and we have taken advantage of it to retrieve a large sample of low-metallicity M dwarfs.

We selected a sub-sample of low-metallicity stars which represent a sequence going from M0 to M9.5 from optical spectra. The objects were specifically chosen to be the (or among the) brightest of their subclass and be observable either in August or January from the Southern hemisphere to achieve the best signal-to-noise ratio possible over a wide wavelength range. We completed our sample with LHS377 [sdM7; @gizis97a], observed with the same telescope/instrument but at higher spectral resolution [@rajpurohit16a]. Our final sample contains 15 sdM0–sdM9.5, 16 esdM0.0–esdM8.5, and 12 usdM0.0–usdM8.5) with almost one object per spectral sub-type. We are missing the sdM7.5, sdM8, and sdM9 in the subdwarf sequence, the esdM2.5 and esdM8 in the 