  The Very High Energy (VHE) gamma-ray emission of the closest BL Lacertae
objects Markarian 501 and Markarian 421 has been observed by the CAT telescope
in 1997 and 1998. In 1997 Mrk 501 exhibited a remarkable series of flares, with
a VHE emission peaking above 250 GeV. The source showed correlated emissions in
the X-ray and VHE gamma-ray bands, together with intensity-spectral hardness
correlation in the latter energy range. During small flares in 1998, Mrk 421
became the second extragalactic source detected by CAT. Its spectral properties
are compared to those of Mrk 501. Theoretical implications for jet astrophysics
are briefly discussed.

Astrophysics