HI absorption in a gravitational lens at z ∼ 0 . 7645

We have used the Westerbork Synthesis Radio Telescope to detect HI 21cm absorption at z ∼ 0.7645 in the gravitational lens system towards PMN J0134−0931. The 21cm profile has two broad components, with peak optical depths of 0.047 ± 0.007 and 0.039 ± 0.007, at heliocentric redshifts 0.76470 ± 0.00006 and 0.76348 ± 0.00006, respectively. The redshift of the stronger component matches that of CaII H and K absorption detected earlier. The absorption has a total velocity width of ∼ 500 km s−1 (between nulls) and an equivalent width of 7.1 ± 0.08 km s−1. This would imply a total HI column density of 2.6 ± 0.3 × 1021 cm−2, for a spin temperature of 200 K and a covering factor of unity. The high estimated HI column density is consistent with the presence of large amounts of dust at the lens redshift; the intervening dust could be responsible for the extremely red colour of the background quasar.