Limits on Dust Extinction in B3 QSOS
The broad range of optical-infrared colours of radio QSOs, 1 < B-K < 6, has been cited as evidence for several mag of dust extinction (Webster et al.. If such large extinctions are typical, the implications for our understanding of the space density of optically selected QSOs are profound. We have previously found that the host galaxies of several of the reddest B3 QSOs are readily detectable in K-band images. This suggests contamination of the K apparent magnitudes by starlight, i.e. the redness in B-K may be due to excess light in K, rather than to dust extinction of the B light. We have now imaged the B3 QSOs in UBVR, and we use the range of observed optical and optical-IR colours to place an upper limit on the amount of dust extinction present, rest-frame A < 1.5 mag.