ARE THERE 2 POPULATIONS OF BL LAC OBJECTS

We present single-epoch infrared photometry of two complete samples of BL Lacertae objects, one radio-selected (RBLs) and one X-ray-selected (XBLs). We find that the RBLs fall into two distinct groups on the (J -H)-(H-K) plane, corresponding to those sources in which the host gala xy makes a significant contribution to the infrared colours and those with higher redshift whose infrared spectra are dominated by power-law non-thermal emission. We also find that the XBLs have colours interme diate between these two groups (although with significant scatter). We show that the infrared colours of Fanaroff-Riley class 1 galaxies (FR 1), XBLs and RBLs are consistent with mixing of stellar emission with an increasing proportion of non-thermal emission, and argue that the F R1s, XBLs and RBLs form part of a single population, with the FR1s and XBLs both being at larger angles to the line of sight than are the RB Ls, and differing only in the luminosities of their non-thermal compon ents.