2009ApJ...694.1559S

This paper presents multiband photometric follow-up observations of the Neptune-mass transiting planet GJ 436b, consisting of five new ground-based transit light curves obtained in 2007 May. Together with one already published light curve, we have at hand a total of six light curves, spanning 29 days. The analysis of the data yields an orbital period P = 2.64386±0.00003 days, midtransit time T_c[HJD] = 2454235.8355±0.0001, planet mass M_p= 23.1 ±0.9 M_⊕= 0.073±0.003 M_Jup, planet radius R_p= 4.2±0.2 R_⊕= 0.37±0.01 R_Jup, and stellar radius R_s= 0.45±0.02 R_☉. Our typical precision for the midtransit timing for each transit is about 30 s. We searched the data for a possible signature of a second planet in the system through transit timing variations (TTV) and variation of the impact parameter. The analysis could not rule out a small, of the order of a minute, TTV and a long-term modulation of the impact parameter, of the order of +0.2/yr.