Limits on dust extinction in B3 QSOS

The broad range of optical-infrared colours of radio QSOs, 1 < B - K < 6, h as been cited as evidence for several mag of dust extinction (Webster et al ., 1995). If such large extinctions are typical, the implications for our u nderstanding of the space density of optically selected QSOs are profound. We have previously found that the host galaxies of several of the reddest B 3 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 U B V R, and we use the range of observed o ptical and optical-IR colours to place an upper limit on the amount of dust extinction present, rest-frame A(V) < 1.5 mag.