DISCOVERY OF NEUTRAL HYDROGEN-21 CENTIMETER ABSORPTION AT REDSHIFT 0.25 TOWARD PKS-1413+135

Strong neutral hydrogen 21 cm absorption has been discovered at z = 0.24671 +/- 0.00001 in the radio spectrum of the BL Lac object PKS 1413 + 135. The implied H I column density is N(H I) = 1.3 x 10(19) x (T(s)/f) cm-2, where T(s) is the spin temperature of the gas and f is the H I covering factor. Given that the source is well resolved on VLBI scales, it is likely that f less-than-or-equal-to 0.1. This observation supports the conclusions of Stocke et al. (1992) that the extremely red spectrum of this BL Lac object in the near-infrared is due to a large amount of extinction and that the " host " galaxy is a spiral of type later than S0.