Homogeneous J, H, K, L, photometry of 42 BL Lacertae objects is presented. The observations cover a period of 3 yr with typical time interval of 6 months. We show that the near-IR nonthermal emission of all the objects but two is well described by a single power law (f_v_ is proportional to v^-α^) after recounting for host galaxy contribution. In spite of large flux variability, changes of spectral index are small and rare. In the few cases where significant variations of α are observed, no evidence of correlation with the flux level is found. Our data confirm that the spectral index distribution is bimodal with the x-ray selected objects being significantly flatter than radio selected ones.
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Bersanelli et al. (1992) studied this question.
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