  The increase in luminosity with time of a main sequence star eventually can
lead to substantial evaporation of the oceans on an orbiting terrestrial
planet. Subsequently, the gas phase water in the planet's upper atmosphere can
be photodissociated by stellar ultraviolet and the resulting atomic hydrogen
then may be lost in a wind. This gaseous envelope may pass in front of the host
star and produce tansient, detectable ultraviolet absorption in the Lyman lines
in systems older than 1 Gyr.
