Here’s the latest update on the changes in the orbital period of WASP-12b, from a new paper by Samuel Yee et al.
The times of transit are getting earlier, which means that the period is decreasing slightly. By also considering the times of occultation (when the planet passes behind the star), and also the radial-velocity measurements of the system, the authors deduce that the changes are not the effect of some other planet, but are a real decay in the orbit of WASP-12b. This is expected to occur as a result of tidal interactions between the planet and its host star.
One notable conclusion is that the rate of period decay in WASP-12b is much faster than that in WASP-19b, which shows no detectable period change yet, despite it being an even shorter-period hot Jupiter, which should increase tidal interactions. Yee et al suggest that the difference could arise if the host star WASP-12 is a sub-giant star, whereas WASP-19 is not.