extreme subdwarf sequence, and more usdM (usdM1.5, usdM2, usdM3.0, usdM6.5, usdM7, and usdM8). We note that these targets were included in our original sample but not observed during the ESO service runs. Table \[tab\_MplusTeff\_sdM:log\_obs\_Xshooter\] lists their coordinates, optical SDSS$i$ magnitudes, optical spectral types on the @lepine07c scheme, dates of observations, mean airmass, exposure times in all three arms, and numbers of AB cycles for all 43 subdwarfs.

VLT/X-shooter spectroscopy {#MplusTeff_sdM:spec_obs}
==========================

We carried out spectroscopy from the UV- to the $K$ band with the X-Shooter spectrograph [@dOdorico06; @vernet11] mounted on the Cassegrain focus of the Very Large Telescope (VLT) Unit 2. Observations were conducted in service mode by the European Southern Observatory (ESO) staff over the course of three semesters, between August 2012 and June 2014 (Table \[tab\_MplusTeff\_sdM:log\_obs\_Xshooter\]). The conditions at the time of the observations met the clear sky request, with an airmass less than 1.6, grey conditions, and a seeing better than 1.2 arcsec. We set the individual on-source integration times according to the magnitudes of the targets and used the multiple AB patterns to correct for the sky contribution (mainly) in the near-infrared. All observations were done with the slit oriented at parallactic angle. We list all 