effective temperatures by at least 100K with a similar interval for temperatures hotter than 3400K, producing ranges of 3700–2700K. The agreement is better at lower temperatures and within the uncertainty of 100K (Tables \[tab\_MplusTeff\_sdM:table\_best\_fits\_LL\] and \[tab\_MplusTeff\_sdM:table\_best\_fits\_LF\]). We also find that the fit to the optical region only yields typically lower effective temperature by 100K to 200K (Table \[tab\_MplusTeff\_sdM:table\_best\_fits\_VV\] in Appendix \[MplusTeff\_sdM:appendix\_model\_fits\]).

![T$_{\rm eff}$ as a function of spectral type for solar-metallicity M dwarfs (black diamonds with line), sdM (red dots), esdM (green triangles) and usdM (blue squares). Error bars on spectral types and model parameters are marked for sdM only for clarity purposes. Error bars on spectral type and temperatures are $\pm$0.25 and $\pm$100K, respectively. The point for the sdM7 subdwarf corresponds to LHS377 from @rajpurohit16a. []{data-label="fig_MplusTeff_sdM:plot_SpT_Teff"}](plot_subdwarfs_SpT_Teff.png){width="\linewidth"}

Gravity scale {#MplusTeff_sdM:Comparison_grav}
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Subdwarfs are old low-mass stars that belong to the thick diskor halo of our Galaxy. On average, they are much older than their solar-metallicity counterparts. @monteiro06a identified several white dwarf-subdwarf systems in the thick disk and derived ages of 6–9 Gyr for two of them with M subdwarf companions. From the comparison with FGK stellar templates selected as benchmarks for the Gaia mission [@jofre14a], @scholz15b inferred a possible age 