of multiple systems composed of a solar-type primary and a M dwarf secondary at optical and infrared wavelengths [@woolf05; @woolf06; @bean06a; @woolf09; @rojas_ayala10; @rojas_ayala12; @muirhead12; @terrien12a; @onehag12; @neves13; @neves14; @hejazi15; @newton15a; @lindgren16a]. These surveys usually focus on slightly metal-poor M dwarfs, with metallicities above $-$1.0 dex. An extension to lower metallicities is needed to complement the precise distances that the Gaia satellite [@deBruijne12] will provide for a great number of spectroscopically-confirmed thick disk and halo M dwarfs [@savcheva14] but also understand the role of metallicity on the molecules and dust grains present in the atmosphere of low mass stars.

Subdwarfs are population II dwarfs which appear bluer than solar-metallicity stars due to the dearth of metals in their atmospheres [@baraffe97]. They exhibit thick disk or halo kinematics, including high proper motions or large heliocentric velocities [@gizis97a]. They belong to the first generations of stars and are important tracers of the chemical enrichment history of the Galaxy. The original classification for M subdwarfs (sdM) and extreme subdwarfs (esdM) developed by @gizis97a has been revised and extended by @lepine07c. A new class of subdwarfs, the ultra-subdwarfs (usdM), has been added to the sdM and esdM. The new scheme is based on a 