Observed wavelength positions of individual lines (dots) drift slowly but systematically between successive exposures. The shifts are common for all spectral lines (trend in dotted red). Superposed (dashed gray) is the expected Rossiter-McLaughlin effect, as measured with an iodine cell at the Keck-1 HIRES spectrometer, using a similar spectral resolution in the same wavelength region (Winn et al. 2005). The wavelength scale shows relative shifts only.