  Aims: Our aim is to measure the time delay between the two gravitationally
lensed images of the z = 1.547 quasar SDSS J1650+4251, in order to estimate the
Hubble constant H_0.
  Methods: Our measurement is based on R-band light curves with 57 epochs
obtained at Maidanak Observatory, in Uzbekistan, from May 2004 to September
2005. The photometry is performed using simultaneous deconvolution of the data,
which provides the individual light curves of the otherwise blended quasar
images. The time delay is determined from the light curves using two very
different numerical techniques, i.e., polynomial fitting and direct
cross-correlation. The time delay is converted into H_0 following analytical
modeling 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 discriminate
well between families of lens models. As for most other multiply imaged
quasars, only models of the lensing galaxy that have a de Vaucouleurs mass
profile plus external shear give a Hubble constant compatible with the current
most popular value (H_0 = 72 +/- 8 km s-1 Mpc-1). A more realistic singular
isothermal sphere model plus external shear gives H_0 = 51.7 +4.0 -3.0 km s-1
Mpc-1.
