1ST DETECTION OF CO (J=0-]1) ABSORPTION AT COSMOLOGICAL DISTANCES (Z=0.247)

We report the first detection of CO absorption lines in an intermediat e redshift galaxy, at z=0.24671. The background continuum source is th e BL Lac object PKS 1413+135, which is likely to be situated in the nu cleus of the spiral galaxy hosting the absorbing clouds. At least thre e CO(J=0-->1) components were detected. The lines are very narrow, dow n to 0.6 km s-1. The filling factor of the absorbing clouds is large ( greater than or similar to 80%). The extent of the continuum source is unknown. VLBI cm data suggests a size of several parsecs, whereas rap id variability suggests a size of approximately 10(3) AU. Since the fi lling factor and optical depth cannot be determined independently, we can only derive a lower limit to the column density of CO. In the case of a diffuse component completely covering the background source, the N(CO) column density is 1 x 10(16) cm-2 and the H-2 column density 2 x 10(20) cm-2, but the true values could be much higher. The CO absorp tion lines change in depth on time scale of a few weeks. We attribute these variations to the superluminal motions of the radio jet in PKS 1 413+135 or to proper motion of very small-scale molecular clumps.