  We present an X-ray diffraction study of the quasi-2D conductors
a-(BEDTTTF)2MHg(SCN), with M=K and Rb. They exhibit a phase transition of the
density wave type at TDW=8-10K and 12-13K respectively, evidenced by
magnetoresistivity, specific heat, NMR and Hall constant measurements. The
structural study shows the presence of satellite reflections already at ambient
temperature. The related modulation is incommensurate with multiple harmonics.
For some of the compounds studied, the intensity of the satellite reflections
strongly increases below TDW. According to Fermi surface (FS) calculations, the
wave vector of the structural modulation achieves a quite good nesting of the
global FS. This suggests a coupling of the modulation with the electronic
degrees of freedom leading to a charge density wave ground state.
