  The Very High Energy (VHE) gamma-ray emission of the BL Lacertae objects
  Markarian 501 and Markarian 421 has been observed by the CAT Imaging
Atmospheric Cherenkov Telescope in 1997 and 1998. The spectrum extraction
method is presented, and the spectral properties of both sources are compared
in different activity states. Theoretical implications for jet astrophysics are
discussed.
