ar X iv : a st ro - p h / 97 08 22 8 v 1 2 5 A ug 1 99 7 Polars – multisite emission – multiwavelength observation

We review the main observational characteristics of AM Herculis stars (polars) at X-ray, EUV, UV, IR and optical wavelengths. Particular emphasis is given to multi-epoch, multi-wavelength observations of the eclipsing polar HU Aqr (RXJ2107.9-0518). In AM Herculis stars the broad-band spectral energy distribution from X-rays to the IR is governed by only very small structures: the hot accretion regions on the footpoints af accreting field lines. The most extended structures in the binary systems on the other hand, the massdonating secondary stars and the accretion streams, distinctly appear only as Doppler-shifted emission or absorption lines. They can best be studied by investigating selected narrow spectral features in the optical, ultraviolet or the near infrared. In this contribution both aspects will be highlighted, the structure of the accretion regions as inferred from multi-wavelength observations with low or no spectral resolution, as well as the structure of the secondary stars and the accretion streams as inferred from high-resolution spectral observations and Doppler mapping. Universitäts-Sternwarte Göttingen, Geismarlandstr. 11, D-37083 Göttingen, Germany South African Astronomical Observatory, PO Box 9, Observatory 7935, Cape Town, RSA Department of Physics and Astronomy, University of Wyoming, P.O. Box 3905, University Station, Laramie, WY, 82071, USA MSSL, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK Univ. of St. Andrews, School of Physics and Astronomy, North Haugh, St. Andrews, Fife KY16