JHK observations of quasars and BL Lac objects

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Abstract
Infrared photometry of 40 QSOs and seven BL Lac objects is presented. In some cases the observations extend over several years. Twelve additional QSOs were not detected at a 3 σ upper limit of K = 14.5. The colours of non-variable QSOs as a function of redshift can be understood in terms of the presence or absence of spectral lines and continua in the various bandpasses. It is shown that the violently variable QSOs have colours indicative of a power law flux distribution, that they are mostly of high luminosity and that they make a major contribution to the scatter present in the infrared Hubble diagram. Some BL Lac objects have colours which clearly reveal the presence of underlying galaxies.

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