Using low-temperature gaseous helium close to the critical point, we investigate the Prandtl-number dependence of the effective heat conductivity (Nusselt number) for a 1/2 aspect ratio Rayleigh-Bénard cell. Very weak dependence is observed in the range 0.7 < Pr < 21; 2 × 10^8 < Ra < 2 × 10^10: the absolute value of the average logarithmic slope δ = (∂ln Nu/∂ln Pr)Ra is smaller than 0.03. A bimodality of Nu, with 7% difference between the two sets of data, is observed, which could explain some discrepancies between precise previous experiments in this range.