A new method for the estimate of

We present a new method to estimate the Hubble constant H0 from the measured time delays in quadruply imaged gravitational lens systems. We show how it is possible to get an estimate of H0 without the need to completely reconstruct the lensing potential thus avoiding any a priori hypothesis on the expression of the galaxy lens model. Our method only needs to assume that the lens potential may be expressed as $r^{\alpha} F(\theta)$, whatever the shape function $F(\theta)$ is, and it is thus able to fully explore the degeneracy in the mass models taking also into account the presence of an external shear. We test the method on simulated cases and show that it does work well in recovering the correct value of the slope α of the radial profile and of the Hubble constant H0. Then, we apply the same method to the real quadruple lenses PG 1115+080 and B1422+231 obtaining $H_0 = 58^{+17}_{-15} \ {\rm km \ s^{-1} \ Mpc^{-1}}$ (68% CL)