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Aims: Our aim is to measure the time delay between the two gravitationallylensed images of the z = 1.547 quasar SDSS J1650+4251, in order to estimate theHubble constant H_0.  Methods: Our measurement is based on R-band light curves with 57 epochsobtained at Maidanak Observatory, in Uzbekistan, from May 2004 to September2005. The photometry is performed using simultaneous deconvolution of the data,which provides the individual light curves of the otherwise blended quasarimages. The time delay is determined from the light curves using two verydifferent numerical techniques, i.e., polynomial fitting and directcross-correlation. The time delay is converted into H_0 following analyticalmodeling of the potential well.  Results: Our best estimate of the time delay is Dt = 49.5 +/- 1.9 days, i.e.,we reach a 3.8% accuracy. The R-band flux ratio between the quasar images,corrected for the time delay and for slow microlensing, is F_A /F_B = 6.2 +/-5%.  Conclusions: The accuracy reached on the time delay allows us to discriminatewell between families of lens models. As for most other multiply imagedquasars, only models of the lensing galaxy that have a de Vaucouleurs massprofile plus external shear give a Hubble constant compatible with the currentmost popular value (H_0 = 72 +/- 8 km s-1 Mpc-1). A more realistic singularisothermal sphere model plus external shear gives H_0 = 51.7 +4.0 -3.0 km s-1Mpc-1.Comment: 8 pages, 12 figures, accepted by A&

COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses