HD  189733b transit light curves obtained for selected spectral lines versus time (or orbital phase): O I (130.4 nm) triplet, C II (133.5 nm) doublet, Si III (120.6 nm), and Si IV (139.8 nm) lines. Usually, those lines are used as a diagnostic of solar flux variability in the FUV (Brekke et al. 1996). A synthetic light curve corresponding to an occulting disk of ~ 1.7 R_p is shown with C II and O I data points. Dark error bars are statistical while the gray bars include the non-statistical scatter of the stellar signal within the given HST orbit.