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Warm Spitzer Occultation Photometry of WASP-26b at 3.6μm and 4.5μm

We present new warm Spitzer occultation photometry of WASP-26 at 3.6μm and 4.5μm along with new transit photometry taken in the g,r and i bands. We report the first detection of the occultation of WASP-26b, with occultation depths at 3.6μm and 4.5μm of 0.00126 +/- 0.00013 and 0.00149 +/- 0.00016 corresponding to brightness temperatures of 1825+/-80K and 1725+/-89K, respectively. We find that the eccentricity of the orbit is consistent with a circular orbit at the 1σ level with a 3σ upper limit of e < 0.04. According to the activity-inversion relation of Knutson et al. (2010), WASP-26b is predicted to host a thermal inversion. The brightness temperatures deduced from the eclipse depths are consistent with an isothermal atmosphere, although it is within the uncertainties that the planet may host a weak thermal inversion. The data are equally well fit by atmospheric models with or without a thermal inversion. We find that variation in activity of solar-like stars does not change enough over the time-scales of months or years to change the interpretation of the Knutson et al. (2010) activity-inversion relation, provided that the measured activity level is averaged over several nights. Further data are required to fully constrain the thermal structure of the atmosphere because the planet lies very close to the boundary between atmospheres with and without a thermal inversion.