THE DUST-TO-GAS RATIO IN THE DAMPED LY-ALPHA CLOUDS TOWARD THE GRAVITATIONALLY LENSED QSO-0957+561

We present HST/FOS1 spectra of the two bright images (A and B) of the gravitationally lensed QSO 0957 + 561 in the wavelength range 2200-330 0 Angstrom. We find that the absorption system (z(abs) = 1.3911) near z(em) is a weak, damped Ly alpha system with strong Ly alpha absorptio n lines seen in both images. However, the H I column densities are dif ferent, with the line of sight to image A intersecting a larger column density. The continuum shapes of the two spectra differ in the sense that the flux level of image A increases more slowly toward shorter wa velengths than that of image B. We explain this as the result of diffe rential reddening by dust grains in the damped Ly alpha absorber. A di rect outcome of this explanation is a determination of the dust-to-gas ratio, k, in the damped Ly alpha system. We derive k = 0.55 +/- 0.18 for a simple 1/lambda extinction law and k = 0.31 +/- 0.10 for the Gal actic extinction curve. For gravitationally lensed systems with damped Ly alpha absorbers, our method is a powerful tool for determining the values and dispersion of k, and the shapes of extinction curves, espe cially in the FUV and EUV regions. We compare our results with previou s work.