[en] The low temperature phase in (EDT-TTF)₂AuBr₂ was investigated by ¹H NMR and magnetic susceptibility measurements for protonated and partially deuterated single crystals. Two sharp peaks were observed in the temperature dependence of T₁^-1 for both salts, associated with the SDW transition T_SDW and the sub-phase transition T* common in the incommensurate SDW phase. The values of T_SDW determined as 15 and 19 K for h- and d-salts, respectively, indicate the negative chemical pressure effect. The obtained ratios T*/T_SDW 0.37 and 0.43 for h- and d-salts, respectively, are slightly larger than the common value T*/T_SDW≅0.3 observed in (TMTCF)₂X. The ratio T*/T_SDW increases for larger T_SDW and the low temperature sub-phase becomes more stable.