Rapid interstellar scintillation of quasar PKS 1257-326

PKS 1257-326 is one of three quasars known to show unusually large and rapid, intra-hour intensity variations, as a result of scintillation in the turbulent Galactic interstellar medium. We have measured time delays in the variability pattern arrival times at the VLA and the ATCA, as well as an annual cycle in the time-scale of variability for this source. Results of the two-station time delay observations are presented here. Implications for the, scintillation of this source axe discussed in the light of these results, together with results from two years of monitoring with the ATCA