  We present new 3.6 and 4.5 micron secondary eclipse measurements for five
cool (less than approximately 1000 K) transiting gas giant planets: HAT-P-15b,
HAT-P-17b, HAT-P-18b, HAT-P-26b, and WASP-69b. We detect eclipses in at least
one bandpass for all planets except HAT-P-15b. We confirm and refine the
orbital eccentricity of HAT-P-17b, which is also the only planet in our sample
with a known outer companion. We compare our measured eclipse depths in these
two bands, which are sensitive to the relative abundances of methane versus
carbon monoxide and carbon dioxide, respectively, to predictions from 1D
atmosphere models for each planet. For planets with hydrogen-dominated
atmospheres and equilibrium temperatures cooler than approximately 1000 K, this
ratio should vary as a function of both atmospheric metallicity and the
carbon-to-oxygen ratio. For HAT-P-26b, our observations are in good agreement
with the low atmospheric metallicity inferred from transmission spectroscopy.
We find that all four of the planets with detected eclipses are best matched by
models with relatively efficient circulation of energy to the nightside. We see
no evidence for a solar-system-like correlation between planet mass and
atmospheric metallicity, but instead identify a potential (1.9 sigma)
correlation between the inferred methane/(carbon monoxide + carbon dioxide)
ratio and stellar metallicity. Our ability to characterize this potential trend
is limited by the relatively large uncertainties in the stellar metallicity
values. Our observations provide a first look at the brightness of these
planets at wavelengths accessible to the James Webb Space Telescope, which will
be able to resolve individual methane, carbon monoxide, and carbon dioxide
bands and provide much stronger constraints on their atmospheric compositions.
