  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 more
than an order of magnitude larger than the quiescent level. Spectral analysis
of the data allows us to study the temperature and emission measure
distribution, and the coronal metal abundance, for the quiescent phase and, in
the case of ROSAT, also during the evolution of the flare. The global coronal
metallicity $Z/Z_{\odot} \sim 0.1$ derived from both ROSAT and ASCA data is
much lower than solar and presumably also much lower than the photospheric
abundance expected for this very young star. The temperature structure of the
quiescent corona was about the same during the various observations, with a
cooler component at $T_1 \sim 7$ MK and a hotter component (to which only ASCA
was sensitive) at $T_2 \sim 20$ MK. During the flare, the low temperature
component remained approximately constant and equal to the quiescent value,
while the high-temperature component was the only one that varied. We have
modeled the flare with the hydrodynamic-decay sustained-heating approach of
Reale 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 the
Sun, but comparable with the typical dimensions inferred for other stellar
flares. We have compared the derived loop size with that estimated with a
simpler (but physically inconsistent) approach, finding that for this, as well
for several other stellar flares, the two methods give comparable loop sizes.
Possible causes and consequences of this result are discussed.
