We present a study of the MgII absorption system at z=0.558 towards the BL Lac object PKS 0118-272 based on high resolution spectra (λ/{DELTA}λ=~2x10^4^)...

A total of 85 UBVRI observations of BL Lac objects is presented. For most of them, an average continuum can be traced by means of well-defined color indices...

UBVRI observations of 37 QSOs are reported. The sample is considered as a good representation of the bright (V not greater than 17) quasars in the compilation...

Context. Because of its proximity, Mrk 421 is one of the best sources on which to study the nature of BL Lac objects. Its proximity allows us to characterize...

We report on a multiband variability and correlation study of the TeV blazar Mrk 421 during an exceptional flaring activity observed from 2013 April 11 to 19...

We present a polarization variability analysis of a sample of 26 γ-ray blazars monitored by the Steward Observatory between 2008 and 2018 in the optical band...

The blazar Mrk 501 was observed at energies above 0.10 TeV with the MAGIC Telescope from 2005 May through July. The high sensitivity of the instrument enabled...

Near infrared monitoring taken with the 1.5 m Carlos Sanchez Telescope in Tenerife of the decline of 3C279 during its 1988 outburst is presented. These data are...

We report here the first results of a program aimed at obtaining high signal to noise (up to ~ 500) optical spectroscopy of a large sample of BL Lac objects...