The far-infrared emission of the radio-loud quasar 3C 318

3C 318, a radio-loud quasar at z = 1 : 574, is a subgalactic-sized radio source, and a good test-bed for the interplay between black hole and galaxy growth in the high-z Universe. Based on its IRAS, ISO, and SCUBA detections, it has long been considered as one of the most intrinsically luminous (L-IR > 10(13) L-circle dot) infrared sources in the Universe. Recent far-infrared data from the Herschel Space Observatory reveal that most of the flux associated with 3C 318, measured with earlier instruments, in fact comes from a bright nearby source. Optical imaging and spectroscopy show that this infrared-bright source is a strongly star-forming pair of interacting galaxies at z = 0 : 35. Adding existing Spitzer and SDSS photometry, we perform a spectral energy distribution analysis of the pair, and find that it has a combined infrared luminosity of L-IR = 1.5 x 10(12) L-circle dot, comparable to other intermediate-redshift ultra-luminous infrared galaxies studied with Herschel. Isolating the emission from 3C 318' s host, we robustly constrain the level of star formation to a value a factor of three lower than that published earlier, which is more in line with the star formation activity found in other Herschel-detected 3CR objects at similar redshift.