Can HO be determined from giant luminous arcs ?

We propose a new way of deriving some constraints on H_0_ from the recently discovered giant luminous arcs. It is now well known that rich clusters of galaxies act as giant gravitational telescopes and can produce elongated images of very distant galaxies located behind them. The gravitational magnification is in some cases large enough to enable us to detect the intrinsic rotational velocity of the source galaxy in an emission line of the spectrum of the arc. An absolute magnitude of the galaxy can then be derived by applying the Tully-Fisher relation. From the corresponding photometric data corrected for gravitational magnification and cosmological redshift, the apparent magnitude of the source is determined. Finally the distance modulus is obtained in terms of H_0_ and q_0_. We apply this method to the "straight arc" observed in the cluster Abell 2390 (z=0.231). By interpreting the velocity gradient detected along the arc in terms of rotation for the background galaxy (at z=0.913) we can discuss our method and the dependence of various parameters on H_0_. With reasonable assumptions we find a preliminary value H_0_=95+/-26 km s^-1^ Mpc^-1^ for q_0_=0 or H_0_=75+/-20 for q_0_=0.5. New observations of velocity gradients in bright arcs at different redshifts could provide stronger constraints on H_0_ as well as a better knowledge of q_0_.