2013ApJ...770...36K

The Apache Point Survey of Transit Lightcurves of Exoplanets (APOSTLE) observed 10 transits of XO-2b over a period of 3 yr. We present measurements that confirm previous estimates of system parameters like the normalized semi-major axis (a/R_{sstarf}), stellar density (ρ_{sstarf}), impact parameter (b), and orbital inclination (i_orb). Our errors on system parameters like a/R_{sstarf} and ρ_{sstarf}have improved by ∼40% compared to previous best ground-based measurements. Our study of the transit times show no evidence for transit timing variations (TTVs) and we are able to rule out co-planar companions with masses ≥ 0.20 M_⊕ in low order mean motion resonance with XO-2b. We also explored the stability of the XO-2 system given various orbital configurations of a hypothetical planet near the 2:1 mean motion resonance. We find that a wide range of orbits (including Earth-mass perturbers) are both dynamically stable and produce observable TTVs. We find that up to 51% of our stable simulations show TTVs that are smaller than the typical transit timing errors (∼20 s) measured for XO-2b, and hence remain undetectable.