Measurements of optical refractive indices (ordinary and extraordinary), as a function of temperature, are reported for the heavy water / SDS molar ratio M-w = 36, with a decanol / SDS molar ratio M-d = 0.324. The nematic biaxial phase emerges from the crystal phase (which occurs below 18 degrees C) into a coexistence region with two uniaxial nematic phases (discotic N-D and cylindrical N-C) , before entrance in the N-C phase. The transitions are discontinuous, and the biaxial phase differs from the coexistence region N-D + N-C . Results indicate changes of the micellar object with temperature, and at the phase transitions.