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We aim at developing an efficient method to search for late-type subdwarfs(metal-depleted dwarfs with spectral types >M5) to improve the currentstatistics. Our objectives are: improve our knowledge of metal-poor low-massdwarfs, bridge the gap between the late-M and L types, determine their surfacedensity, and understand the impact of metallicity on the stellar and substellarmass function.  We carried out a search cross-matching the SDSS, 2MASS, and UKIDSS usingSTILTS, Aladin, and Topcat. We considered different photometric and propermotion criteria for our selection. We identified 29 and 71 late-type subdwarfcandidates in each cross-correlation over 8826 and 3679 square degrees,respectively. We obtained low-resolution optical spectra for 71 of ourcandidates with GTC, NOT, and VLT and retrieved spectra for 30 candidates fromthe SDSS spectroscopic database. We classified 92 candidates based on 101optical spectra using two methods: spectral indices and comparison withtemplates of known subdwarfs.  We confirmed 86% and 94% of the candidates as late-type subdwarfs from theSDSS vs 2MASS and SDSS vs UKIDSS cross-matches, respectively. These subdwarfshave spectral types ranging between M5 and L0.5 and SDSS magnitudes in ther=19.4-23.3 mag range. Our new late-type M discoveries include 49 subdwarfs, 25extreme subdwarfs, six ultrasubdwarfs, one subdwarf/extreme subdwarf, and twodwarfs/subdwarfs. We derived a surface density of late-type subdwarfs of0.040$^{+0.012}_{-0.007}$ per square degree in the SDSS DR7 vs UKIDSS LAS DR10cross-match. We also checked the AllWISE photometry of known and new subdwarfsand found that mid-infrared colours of M subdwarfs do not appear to differ fromtheir solar-metallicity counterparts of similar spectral types. However, theJ-W2 and J-W1 colours are bluer for lower metallicity dwarfs. (abstractstrongly abridged)

astronomy and astrophysics

New ultracool subdwarfs identified in large-scale surveys using Virtual  Observatory tools: II. SDSS DR7 vs UKIDSS LAS DR6, SDSS DR7 vs UKIDSS LAS  DR8, SDSS DR9 vs UKIDSS LAS DR10, and SDSS DR7 vs 2MASS