  Transit light curves for stellar continua have only one minimum and a "U"
shape. By contrast, transit curves for optically thin chromospheric emission
lines can have a "W" shape because of stellar limb-brightening. We calculate
light curves for an optically thin shell of emission and fit these models to
time-resolved observations of Si IV absorption by the planet HD209458b. We find
that the best fit Si IV absorption model has R_p,SIV/R_*= 0.34+0.07-0.12,
similar to the Roche lobe of the planet. While the large radius is only at the
limit of statistical significance, we develop formulae applicable to transits
of all optically thin chromospheric emission lines.
