An effective vacuum refractive index from gravity and the present ether-drift experiments

Re-analyzing the data published by the Berlin and Duesseldorf ether-drift experiments, we have found a clean non-zero daily average for the amplitude of the signal. The two experimental values, A_0\sim (10.5 \pm 1.3) 10^{-16} and A_0\sim (12.1\pm 2.2) 10^{-16}$ respectively, are entirely consistent with the theoretical prediction (9.7\pm 3.5) 10^{-16} that is obtained once the Robertson-Mansouri-Sexl anisotropy parameter is expressed in terms of N_{vacuum}, the effective vacuum refractive index that one would get, for an apparatus placed on the Earth's surface, in a flat-space picture of gravity .