A Determination of H 0 with the CLASS Gravitational LensB 1608 + 656 : I .

We present the results of a program to monitor the four-image gravitational lens B1608+656 with the VLA. The system was observed over a seven month period from 1996 October to 1997 May. The 64 epochs of observation have an average spacing of 3.6 d. The light curves of the four images of the background source show that the ux density of the background source has varied at the 5% level. We measure time delays in the system based on common features that are seen in all four light curves. The three independent time delays in the system are found to be t BA = 317 d, t BC = 367 d, and t BD = 76 +9 ?10 d at 95% conndence. The uncertainties on the time delays are determined by Monte Carlo simulations which use fake light curves that have the characteristics of the observed light curves. This is the rst gravitational lens system for which three independent time delays have been measured. A companion paper presents a mass model for the lensing galaxy which correctly reproduces the observed image positions, ux density ratios, and time delay ratios. The last condition is crucial for determining H 0 with a four-image lens. We combine the time delays with the model to obtain a value for the Hubble constant of H 0 = 59 +8 ?7 km s ?1 Mpc ?1 at 95% conndence (statistical) for ((M ;) = (1; 0). In addition, there is an estimated systematic uncertainty of 15 km s ?1 Mpc ?1 from uncertainties in modeling the radial mass prooles of the lensing galaxies. The value of H 0 presented in this paper is comparable to recent measurements of H 0 from the