Gas density at outer boundary as a function of time for selected models with a stellar mass of 0.75 M_⊙. The left column displays the SDDO values, the right one the SPL values. Stellar parameters, T_eff, log L, carbon excess class and piston amplitude, are given in the SPL panels. The densities are given in cgs units: T_eff in K, log L in L_⊙, and piston amplitudes in km s⁻¹. We use a fixed range in gas density in all panels.