it is smaller that that found for flat spectrum radio quasars, suggesting there is a difference in the intrinsic brightness of the nuclear source or in the Doppler beaming factor between the two types of blazars.'
author:
- 'Jari K. Kotilainen , Renato Falomo and Riccardo Scarpa'
date: 'Accepted May 18, 1998; received April 10, 1998; in original form February 21, 1998'
title: 'The host galaxies of BL Lac objects in the near–infrared[^1]'
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Introduction
============

BL Lac objects are active galactic nuclei (AGN) characterized by strong and rapidly variable continuum emission and polarization across the electromagnetic spectrum, strong compact flat spectrum radio emission and superluminal motion (see Kollgaard 1992 for general references). They share many properties with flat spectrum radio quasars (FSRQ) and are often grouped together as blazars. The clearest difference between them is that the latter have strong broad emission lines, while these are very weak or absent in BL Lacs.

Blazar properties are usually explained by the beaming model (Blandford & Rees 1978), where the observed emission is dominated by a synchrotron emitting relativistically boosted jet oriented close to our line–of–sight. 