I present the discovery of the first wide M + L extreme subdwarf binary system Gaia J0452 - 36AB .
The binary is located at a distance of 137.27 ^ { +0.68 } _ { -0.67 } pc with a projected separation of 15828 \pm 78 au .
I classified Gaia J0452 - 36AB as esdM1 and esdL0 subdwarfs , respectively .
Gaia J0452 - 36AB have typical halo kinematics , metallicity of [ Fe/H ] \approx - 1.4 , and temperature of \sim 3550 and 2600 K , respectively .
Gaia J0452 - 36AB is a pair of very low-mass stars with masses of 0.151 ^ { +0.029 } _ { -0.019 } and 0.0855 ^ { +0.0014 } _ { -0.0010 } M _ { \sun } , and is a gravitationally bound system .
I tested the metallicity consistency of existing M subdwarf classification schemes with Gaia J0452 - 36AB and a sample of M and L subdwarfs with known metallicity .
I found that the metallicity of each M subclass defined by the the metallicity index \zeta _ { CaH / TiO } is not consistent from mid-to-late M subtypes .
Because late-type M and L subdwarfs have dusty atmospheres and high surface gravity which have significant impacts on CaH and TiO indices that used in the classification .
The metallicity scale of late-type M subdwarfs would be overestimated by the \zeta _ { CaH / TiO } index .
I discussed the mass range of M subdwarfs , and explained the lack of late-type M extreme and ultra subdwarfs , and decreasing binary fraction from sdM , to esdM , and usdM subclasses .
The four M subclasses have different mass ranges .
The comparison between M subclasses is between populations in different mass ranges .
I also present the discovery of Ruiz 440-469B , an M8 dwarf wide companion to a cool DA white dwarf , Ruiz 440-469 .