THE TIME DELAY OF THE QUADRUPLE QUASAR RX J0911.4�0551 1

We present optical light curves of the gravitationally lensed components A ({A1�A2�A3) and B of the quadruple quasar RX J0911.4�0551 ( z p 2.80). The observations were primarily obtained at the Nordic Optical Telescope between 1997 March and 2001 April and consist of 74 I-band data points for each component. The data allow the measurement of a time delay of 146 � 8 days (2 j) between A and B, with B as the leading component. This value is significantly shorter than that predicted from simple models and indicates a very large external shear. Mass models including the main lens galaxy and the surrounding massive cluster of galaxies at, responsible for the external shear, yield (random, 2 j) � 8 (systematic) km s �1 Mpc �1 z p 0.77 H0 p 71 � 4 The systematic model uncertainty is governed by the surface-mass density (convergence) at the location of the multiple images. Subject headings: cosmology: observations — dark matter — distance scale — gravitational lensing — quasars: individual (RX J0911.4�0551) On-line material: color figures 1.

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