The optical to near-infrared emission of BL Lac objects - Simultaneous observations

We present simultaneous optical to near-IR observations of 34 blazars, most of which classical BL Lacertae objects, with repeated observations for 21 of them. The observations cover a period of 3 years with a typical time interval of 6 months. With the sole exception of PKS 2208 - 137, all observed optical to near-IR flux distributions can be described either by a single power law (f_v_ is proportional to v^-α^) or by a power law plus the contribution of a standard elliptical galaxy. The optical and near-IR emission are strongly correlated and usually maintain the spectral index even under substantial flux variations. The average spectral index of the nonthermal emission, in the range 8 X 10^13^ to 7 X 10^14^ Hz, is 1.08 +/- 0.06 with significantly flatter spectra for the x-ray selected subsample.