Cool Stars 14 - Submitted Abstract # 27 This version created on 05 October 2006 Multi-wavelength observations of a giant flare on CN Leo Carolin Liefke, Hamburger Sternwarte Juergen H.M.M. Schmitt, Hamburger Sternwarte Birgit Fuhrmeister, Hamburger Sternwarte Ansgar Reiners, Max-Planck-Institut fuer Sonnensystemforschung We present simultaneous observations of a huge flare on the M5.5 dwarf CN Leo with VLT/UVES and XMM-Newton, with flux increases by factors up to 500 in the optical and up to 100 in X-rays and lasting for approximately 25 minutes. Photometry with a high time resolution (in the order of magnitude of 1 sec) is obtained in X-rays, in the U band with the XMM Optical Monitor operating in Fast Mode, and with the UVES blue and red exposuremeters. Time-resolved spectroscopy in X-rays demonstrates the development of temperature and density of the coronal flaring plasma. The behaviour of the chromosphere is simultaneously traced by various emission lines observed in the blue UVES spectra. Large increases in the line fluxes are accompanied by significant enhancements of the continuum level, mainly in the blue part but also in the red part of the spectra. The red Ca II triplet and lines of the hydrogen Paschen series are observed in emission during the flare. Together with the Balmer lines they trace properties of chromospheric and transition region plasmas. Additionally, magnetically sensitive lines of FeH are suitable to measure flare-related changes in CN Leo's magnetic field. ----------------------------------