The Connection between BL Lacs and FSRQs

We discuss the features that mark the Flat Spectrum Radio Quasars from the BL Lacertae objects. We propose that FSRQs exceeding L ∼ 10 46 erg s −1 are powered by black hole accreting at rates ˙ m ∼ 1 ÷ 10; their power is dominated by the disk components, thermal and BZ. Instead, sources accreting at rates ˙ m ∼ 10 −2 ÷ 10 −3 radiate in the BL Lac mode; here the power is mainly non-thermal, and is driven by the rotational energy stored in a Kerr hole which sustains L ∼ < 10 46 erg s −1 in the jet frame for Gyrs. The two populations may be even linked if around the same objects ˙ m drops in time; then some negative cosmological evolution is expected for the newborn BL Lacs fed by the dying FSRQs. Further implications are discussed.