An optical time delay for the double gravitational lens system FBQ 0951+2635

We present optical R-band light curves of the double gravitationally lensed quasar FBQ 0951+2635 from observations obtained at the Nordic Optical Telescope between March 1999 and June 2001. A time delay of Deltatau = 16 +/- 2 days (1sigma) is determined from the light curves. New constraints on the lensing geometry are provided by the position and ellipticity of the lensing galaxy. For a (Omega(m), Omega(Lambda)) = (0.3, 0.7) cosmology, the time delay yields a Hubble parameter of H-0 = 60(-7)(+9) (random, 1sigma) +/-2 (systematic) km s(-1) Mpc(-1) for a singular isothermal ellipsoid model and H-0 = 63(-7)(+9) (random, 1sigma) +/-1 (systematic) km s(-1) Mpc(-1) for a constant mass-to-light ratio model. In both models, the errors are mainly due to the time-delay uncertainties. Non-parametric models yield H-0 = 64(-7)(+9) (random, 1sigma) +/- 14 (systematic) km s(-1) Mpc(-1).