Photometric follow-up observations of the transiting neptune-mass planet GJ 436b

This paper presents multiband photometric follow-up observations of the Neptune-mass transiting planet GJ436b, 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^-1.