c © ESO 2003 Astronomy

Abstract. We present the discovery of a dusty quasar, SMM J04135+10277, located behind the galaxy cluster Abell 478. The quasar was discovered as the brightest source in a submillimetre survey of high redshift galaxies lensed by foreground rich clusters of galaxies in a project aimed at studying the cosmic star formation history of dusty galaxies. With submillimetre fluxes of S 850 = 25 ± 2.8 mJy and S 450 = 55 ± 17 mJy this object is one of the brightest submillimetre sources known. Optical imaging revealed a point source with I = 19.4 ± 0.1 mag (corrected for galactic extinction). Follow-up optical spectroscopy showed this object to be a quasar at redshift z = 2.837 ± 0.003. The quasar was also detected at shorter infrared wavelengths with the Infrared Space Observatory. This object is the first quasar discovered by its submillimetre emission. Given the general lack of overlap between deep submillimetre and X-ray samples, usually interpreted as a low incidence of active nuclei in submillimetre samples, this is an unusual object. Analysis of number counts of quasars and of submillimetre galaxies bears out this suggestion. We compare the properties of SMM J04135+10277 to those of optically selected quasars with submillimetre emission, and argue that the optical faintness results from a large viewing angle with the direction of relativistic beaming, and not from abnormally high extinction. We also find indications that the bulk of the submillimetre flux density is not powered by the quasar nucleus. This conclusion is supported by analysis of the infrared spectral energy distribution. These results are consistent with previous observations that quasars at higher redshift tend to have a more prominent cold dust component, most likely powered by extended star formation in the host galaxy. The temperature for the cold dust component is found b