Spectral slope variability of BL Lacertae objects in the optical band

Light curves of eight BL Lac objects in the BVRI bands have been analyzed. All of the objects tend to be bluer when brighter. However, spectral slope changes differ quantitatively from those of a sample of QSOs analyzed in a previous paper by Trevese & Vagnetti and appear to be consistent with a different nature of the optical continuum. A simple model representing the variability of a synchrotron component can explain the spectral changes. Constraints on a possible thermal accretion disk component contributing to the optical luminosity are discussed