Difference between the mean RV from ELODIE and from SOPHIE, Δ(RV)_E−S as a function of B − V for a sample of stable stars. The error bars correspond to the quadratic sum of the standard deviation in the ELODIE and SOPHIE RV data. The green solid line is the best linear fit for stars correlated with a G2 mask (black squares). Those with B − V > 0.75 are discarded (red triangles). The black dashed line is the best linear fit for stars correlated with a K5 mask (blue circles). The detection of the slope is insignificant and a constant value is chosen (green dashed line).