  We have analyzed Warm Spitzer/IRAC observations of the secondary eclipses of
three planets, XO-4b, HAT-P-6b and HAT-P-8b. We measure secondary eclipse
amplitudes at 3.6{\mu}m and 4.5{\mu}m for each target. XO-4b exhibits a
stronger eclipse depth at 4.5{\mu}m than at 3.6{\mu}m, which is consistent with
the presence of a temperature inversion. HAT-P-8b shows a stronger eclipse
amplitude at 3.6{\mu}m, and is best-described by models without a temperature
inversion. The eclipse depths of HAT-P-6b can be fitted with models with a
small or no temperature inversion. We consider our results in the context of a
postulated relationship between stellar activity and temperature inversions and
a relationship between irradiation level and planet dayside temperature, as
discussed by Knutson et al. (2010) and Cowan & Agol (2011), respectively. Our
results are consistent with these hypotheses, but do not significantly
strengthen them. To measure accurate secondary eclipse central phases, we
require accurate ephemerides. We obtain primary transit observations and
supplement them with publicly available observations to update the orbital
ephemerides of the three planets. Based on the secondary eclipse timing, we set
upper boundaries for e cos(\omega) for HAT-P-6b, HAT-P-8b and XO-4b and find
that the values are consistent with circular orbits.

Earth and Planetary Astrophysics