In the July 2009 issue of Sky & Tel, there's a neat article on searching not just for exoplanets, but their smaller, and possibly habitable (like Europa?) moons. Contingent on the geometry of a distant solar system being edge on, a transit by an exoplanet will shadow the distant star, leaving a light curve photometric dip. But, in the instances where the putative exoplanet also has a moon, it's the center of mass that obeys Kepler's Laws so nicely, so the clockwork regularity expected for successive transits will be off a little bit, oscillating a little between too early and too late, depending on the relative positions of the two (exoplanet + moon) bodies as the transit starts. This the author, David Kipping, calls Transit Timing Variation...or TTV.
In addition, the duration of the transit will be similarly affected...this he calls Transit Duration Variation...or TDV. As he specializes in this and is working towards his PhD at University College in London, he offers his website for a demo. neat. pete