1995A&A...299..339M

The results of several photometric campaigns in the near IR on the six bright, radio selected BL Lac objects 3C 66A, PKS 0422+004, PKS 0735+178, PKS 0754+100, PKS 0829+046 and OQ 530 are presented. The range of the recorded intensity variations differ from one object to the other, and goes from a minimum of 0.5 to a maximum of 1.6 magnitudes. Spectral index variations were found in all objects but PKS 0754+100. These variations, plotted against the flux level, show in general a rather ample scatter. Despite the scatter, in two objects, 3C 66A and PKS 0829+046, evidence (>95% significance) is found of a positive correlation between the spectral index and the flux in the H band. Since the recorded range of intensity variations in these two objects is the largest in the sample, it is argued (with reference also to other cases previously reported) that the same type of correlation might well be a general property of radio selected BL Lac, whose confirmation would require an appropriately wide dynamic range in the recorded flux levels. The case of two objects in the sample is used to argue that the slope of the correlation is probably different from one object to the other. The implications for the physical models of these sources are briefly discussed.