  This paper presents multi-band photometric follow-up observations of the
Neptune-mass transiting planet GJ 436b, consisting of 5 new ground-based
transit light curves obtained in May 2007. Together with one already published
light curve we have at hand a total of 6 light curves, spanning 29 days. The
analysis of the data yields an orbital period P = 2.64386+-0.00003 days,
mid-transit time T_c [HJD] =2454235.8355+-0.0001, planet mass M_p = 23.1+-0.9
M_{\earth} = 0.073+-0.003 M_{Jup}, planet radius R_p = 4.2+-0.2 R_{\earth} =
0.37+-0.01 R_{Jup} and stellar radius R_s = 0.45+-0.02 R_{\sun}. Our typical
precision for the mid transit timing for each transit is about 30 seconds. 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 year^{-1}.
