Radial velocity, transit & occultation of WASP-4

Additionally, we find that the transit timing variations of all of the WASP-4b transits cannot be explained by the second planet but can be explained with either a decaying orbit or apsidal precession, with a slight preference for orbital decay. Assuming the decay model is correct, we find an updated period of 1.338231587{+/-}0.000000022days, a decay rate of -7.33{+/-}0.71ms/yr, and an orbital decay timescale of {tau}=P/|P.|=15.77{+/-}1.57Myr.

If the observed decay results from tidal dissipation, we derive a modified tidal quality factor of Q_*_'=5.1{+/-}0.9x10^4^, which is an order of magnitude lower than values derived for other hot Jupiter systems. However, more observations are needed to determine conclusively the cause of WASP-4b's changing orbit and to confirm the existence of an outer companion.