The Near-IR-Optical-UV Emission of BL Lacertae Objects

Near--infrared, optical and ultraviolet quasi--simultaneous observations of 11 BL Lacertae objects are reported. For all but one source the dereddened spectral flux distribution in the $8\cdot10^{13}-2\cdot10^{15}$ Hz frequency range can be described by a single power law f$_\nu \propto \nu^{-\alpha}$ with average spectral index $<\alpha>$ = 0.88 $\pm$ 0.42 (standard deviation) plus, where relevant, the contribution of the host galaxy. In most cases the non simultaneous soft X--ray fluxes obtained by the {\it Einstein Observatory} lie on or below the extrapolation of the power law. The results are compared with the average spectral properties of other samples of BL Lacs studied separately in the IR--optical and in the UV bands. The implications for existing models of the objects are shortly discussed.