Context :

Aims : We aim at developing an efficient method to search for late-type subdwarfs ( metal-depleted dwarfs with spectral types \geq M5 ) to improve the current statistics .
Our objectives are : improve our knowledge of metal-poor low-mass dwarfs , bridge the gap between the late-M and L types , determine their surface density , and understand the impact of metallicity on the stellar and substellar mass function .

Methods : We carried out a search cross-matching the Sloan Digital Sky Survey ( SDSS ) Data Release 7 ( DR7 ) and the Two Micron All Sky Survey ( 2MASS ) , and different releases of SDSS and the United Kingdom InfraRed Telescope ( UKIRT ) Infrared Deep Sky Survey ( UKIDSS ) using STILTS , Aladin , and Topcat developed as part of the Virtual Observatory tools .
We considered different photometric and proper motion criteria for our selection .
We identified 29 and 71 late-type subdwarf candidates in each cross-correlation over 8826 and 3679 square degrees , respectively ( 2312 square degrees overlap ) .
We obtained our own low-resolution optical spectra for 71 of our candidates .
: 26 were observed with the Gran Telescopio de Canarias ( GTC ; R \sim 350 , \lambda \lambda 5000–10000 Å ) , six with the Nordic Optical Telescope ( NOT ; R \sim 450 , \lambda \lambda 5000–10700 Å ) , and 39 with the Very Large Telescope ( VLT ; R \sim 350 , \lambda \lambda 6000–11000 Å ) .
We also retrieved spectra for 30 of our candidates from the SDSS spectroscopic database ( R \sim 2000 and \lambda \lambda 3800–9400 Å ) , nine of these 30 candidates with an independent spectrum in our follow-up .
We classified 92 candidates based on 101 optical spectra using two methods : spectral indices and comparison with templates of known subdwarfs .

Results : We developed an efficient photometric and proper motion search methodology to identify metal-poor M dwarfs .
We confirmed 86 % and 94 % of the candidates as late-type subdwarfs from the SDSS vs 2MASS and SDSS vs UKIDSS cross-matches , respectively .
These subdwarfs have spectral types ranging between M5 and L0.5 and SDSS magnitudes in the r = 19.4–23.3 mag range .
Our new late-type M discoveries include 49 subdwarfs , 25 extreme subdwarfs , six ultrasubdwarfs , one subdwarf/extreme subdwarf , and two dwarfs/subdwarfs .
In addition , we discovered three early-L subdwarfs to add to the current compendium of L-type subdwarfs known to date .
We doubled the numbers of cool subdwarfs ( 11 new from SDSS vs 2MASS and 50 new from SDSS vs UKIDSS ) .
We derived a surface density of late-type subdwarfs of 0.040 ^ { +0.012 } _ { -0.007 } per square degree in the SDSS DR7 vs UKIDSS LAS DR10 cross-match ( J = 15.9–18.8 mag ) after correcting for incompleteness .
The density of M dwarfs decreases with decreasing metallicity .
We also checked the Wide Field Survey Explorer ( AllWISE ) photometry of known and new subdwarfs and found that mid-infrared colours of M subdwarfs do not appear to differ from their solar-metallicity counterparts of similar spectral types .
However , the near-to-mid-infrared colours J - W 2 and J - W 1 are bluer for lower metallicity dwarfs , results that may be used as a criterion to look for late-type subdwarfs in future searches .

Conclusions:0