  Many of the known hot Jupiters are formally unstable to tidal orbital decay.
The only hot Jupiter for which orbital decay has been directly detected is
WASP-12, for which transit timing measurements spanning more than a decade have
revealed that the orbital period is decreasing at a rate of $dP/dt\approx
10^{-9}$, corresponding to a reduced tidal quality factor of about $2\times
10^5$. Here, we present a compilation of transit-timing data for WASP-12 and
eleven other systems which are especially favorable for detecting orbital
decay: KELT-16; WASP-18, 19, 43, 72, 103, 114, and 122; HAT-P-23; HATS-18; and
OGLE-TR-56. For most of these systems we present new data that extend the time
baseline over which observations have been performed. None of the systems
besides WASP-12 displays convincing evidence for period changes, with typical
upper limits on $dP/dt$ on the order of $10^{-9}$ or $10^{-10}$, and lower
limits on the reduced tidal quality factor on the order of $10^5$. One possible
exception is WASP-19, which shows a statistically significant trend, although
it may be a spurious effect of starspot activity. Further observations are
encouraged.
