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We discuss ROSAT and ASCA observations of the young active star Gl355}.During the ROSAT observation a strong flare was detected with a peak flux morethan an order of magnitude larger than the quiescent level. Spectral analysisof the data allows us to study the temperature and emission measuredistribution, and the coronal metal abundance, for the quiescent phase and, inthe case of ROSAT, also during the evolution of the flare. The global coronalmetallicity $Z/Z_{\odot} \sim 0.1$ derived from both ROSAT and ASCA data ismuch lower than solar and presumably also much lower than the photosphericabundance expected for this very young star. The temperature structure of thequiescent corona was about the same during the various observations, with acooler component at $T_1 \sim 7$ MK and a hotter component (to which only ASCAwas sensitive) at $T_2 \sim 20$ MK. During the flare, the low temperaturecomponent remained approximately constant and equal to the quiescent value,while the high-temperature component was the only one that varied. We havemodeled the flare with the hydrodynamic-decay sustained-heating approach ofReale at al. (1997) and we have derived a loop semi--length of the order of$\sim 1.5$ stellar radii, i.e. much larger than the dimensions of flares on theSun, but comparable with the typical dimensions inferred for other stellarflares. We have compared the derived loop size with that estimated with asimpler (but physically inconsistent) approach, finding that for this, as wellfor several other stellar flares, the two methods give comparable loop sizes.Possible causes and consequences of this result are discussed.Comment: A&A, in pres

Quiescent and flare analysis for the chromospherically active star Gl 355 (LQ Hya)