A search for distant radio-loud quasars in the CLASS survey: three new radio-selected quasars at z > 4

We report on the search for distant radio-loud quasars in the Cosmic Lens A ll Sky Survey (CLASS) of flat spectrum radio sources with S-5GHz > 30 mJy. Unresolved optical counterparts were selected from APM scans of POSS-I plat es, with e < 19.0 and red o - e > 2.0 colours, in an effective area of simi lar to 6400 deg(2). Four sources were found to be quasars with z > 4, of wh ich one was previously known. This sample bridges the gap between the stron g radio surveys with S-5GHz > 200 mJy and the samples of radio-weak quasars that can be generated via radio observations of optically selected quasars . In addition, four new quasars at z > 3 have been found. The selection cri teria result in a success-rate of similar to 1:7 for radio-loud quasars at z > 4, which is a significant improvement over previous studies. This searc h yields a surface density of 1 per 1600 deg(2), which is about a factor of similar to 15 lower than that found in a similar search for radio-quiet qu asars at z > 4. The study presented here is strongly biased against quasars beyond z > 4.5, since the e-passband of the POSS-I only samples the spectr a shortward of 1200 Angstrom at these redshifts.