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We propose to measure the mass of the isolated M subdwarf 2MASS J13055171-7218081 (hereafter J1305) comma using both photometric and astrometric microlensing techniques. Searches of the recent Gaia DR2 have led to predictions of over 100 close passages of nearby stars in front of background sources that will occur in the next 20 years. Among these comma J1305 is the closest of all comma predicted to have a remarkably small impact parameter of virgul5.8 mas comma occuring in 2019 November. As J1305 passes in front comma it will cause a relativistic deflection of the background star by virgul0.9 milliarcsec comma and a photometric brightening by virgul5.5%. The deflections and the amplifications comma measured with HST.s WFC3 comma will provide the first opportunity to derive a lens mass through BOTH photometric and astrometric microlensing methods. The estimated accuracy of our mass measurement is virgul4%. J1305 is of special interest as an extreme halo M subdwarf comma having a transverse velocity of 236 km-s comma and lying 3 magnitudes below the Population I main sequence. Spectroscopic observations show it to be an M3 dwarf with a very low metallicity of CORCHETE_OPENFe-HCORCHETE_CLOSE = -1.8. Its estimated mass is virgul0.11 Msun comma but this is exquisitely sensitive to luminosity comma effective temperature comma and metallicity. To date comma there are only a small number of mass measurements comma based on binary stars comma for metal-deficient M dwarfs comma and none at all for masses less than 0.17 Msun or for CORCHETE_OPENFe-HCORCHETE_CLOSE < -0.9. Our program will provide the first direct comma model-independent mass determination for an extremely metal-deficient M dwarf comma and a critical test of theoretical models for metal poor comma low-mass stars.