2007A&A...468..221F

We present simultaneous observations with VLT/UVES and XMM-Newton of the active M5.5 dwarf CN Leo (Gl 406). The data were gathered during three half nights in May 2004 and December 2005, and they cover quiescent states, as well as flaring activity. Our main aim is to derive coronal properties from X-ray data and to compare these to results from the optical FeXIII line. We studied simultaneously-measured coronal and chromospheric parameters of CN Leo as determined from the XMM-Newton X-ray data, the forbidden optical coronal FeXIII line at 3388Å, and various optical chromospheric emission lines. We find that different activity levels of CN Leo can be traced as well in X-rays as with the FeXIII line. Moreover, the FeXIII line flux is in good agreement with a prediction using the differential emission measure as determined from the X-ray spectrum and Fe atomic data. We also present coronal X-ray properties for the quiescent and flaring states of CN Leo. During a flare two HeII transition region lines are also detected in the optical data.