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The non-thermal jet emission in active galactic nuclei covers several orders of magnitude in the frequency range. Hence the observational approach needs multi-wavelength (MWL) campaigns collecting data in the radio, optical, UV, X-rays, high energy until the Very High Energy (VHE) gamma-ray band. MAGIC, a system of two 17 m diameter telescopes at

We present new optical spectroscopy of the γ-ray binary LS 5039. Our data show evidence for sub-orbital modulation in the radial velocities with amplitude ∼ 7 km/s and period ∼ Porb/4. This short-term oscillation is stable over at least 7 years and it is likely triggered by non-radial oscillations of the O6.5V optical star. We also present the

We are performing an unprecedentedly long and dense monitoring of the multi-frequency (radio to TeV) emission from the classical TeV blazars Mkn 421 and Mkn 501. These objects are among the brightest X-ray/TeV blazars in the sky and among the few sources whose spectral energy distributions (SED) can be completely characterised by the current

Observations and detections of Active Galactic Nuclei (AGN) by Cherenkov telescopes are often triggered by information about high flux states in other wavelength bands. To overcome this bias, the VHE gamma-ray telescope MAGIC has conducted dedicated monitoring observations of nearby AGN since 2006. Three well established, TeV-bright blazars were

Multiple images of quasars formed by gravitational lensing are typically seen through the lensing galaxy. The granulation of the galaxy in stars or other compact objects induces local (`microscopic') but strong fluctuations in the gravitational potential that subdivide each image in several micro-images changing the expected flux of the image (flux