Comparison of radial velocities for the stars derived from the combined spectrum and from the individual spectra separately and then averaged (see Sect. 2.2.3). The x-axis shows the value derived from the combined spectra whilst the y-axis shows the difference between v_r determined in the two ways. The error-bars for the difference is the coadded errors estimated from IRAF (for the combined spectra) and the σ around the mean for the independent measures. Errors on the x-axis are too small to be seen.