We present optical photometry of 16 transits of the super-Earth GJ?1214b, allowing us to refine the system parameters and search for additional planets via transit timing. Starspot-crossing events are detected in two light curves, and the star is found to be variable by a few percent. Hence, in our analysis, special attention is given to systematic errors that result from starspots. The planet-to-star radius ratio is 0.11610 ± 0.00048, subject to a possible upward bias by a few percent due to the unknown spot coverage. Even assuming this bias to be negligible, the mean density of the planet can be either 3.03 ± 0.50?g?cm–3 or 1.89 ± 0.33?g?cm–3, depending on whether the stellar radius is estimated from evolutionary models, or from an empirical mass-luminosity relation combined with the light curve parameters. One possible resolution is that the orbit is eccentric (e ≈ 0.14), which would favor the higher density, and hence a much thinner atmosphere for the planet. The transit times were found to be periodic within about 15 s, ruling out the existence of any other super-Earths with periods within a factor of two of the known planet.
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