The corona of the dMe flare star AD Leo

We have systematically studied the X-ray emission (both the quiescent compo nent and the hares) of the dM3e star AD Lee, analyzing the existing observa tions from the Einstein IPC, ROSAT PSPC and ASCA SIS instruments. Using a c onsistent method which explicitly considers the possibility of sustained he ating we have analyzed the six flares which have sufficient statistics, der iving constraints on the physical parameters of the flaring regions. In all cases the flaring loops are likely compact (L similar or equal to 0.3 R*), and confined to a rather narrow range of sizes, incompatible with the larg e (L greater than or similar to R*) tenuous loops claimed by previous analy ses of flares on AD Leo and other similar stars. The flaring loops appear t o have a larger cross section (beta = r/L similar or equal to 0.3) than cus tomarily assumed (e.g. beta similar or equal to 0.1). An flares show eviden ce of significant heating during the decay phase. Although the derived peak pressures are high (up to P similar or equal to 10(4) dyne cm(-2)) with a peak temperature of similar or equal to 50 MK, the magnetic fields required to confine such loops and to produce the observed flare luminosity are rel atively modest (B similar or equal to 1-2 kG) and fully compatible with the photospheric magnetic fields measured in several flare stars. If the narro w range of loop sizes obtained is extrapolated to the quiescent structures responsible for the active corona, the latter can be naturally scaled up fr om the solar case through a modest (a factor of 10) increase in pressure in otherwise solar-like active structures with a small surface filling factor (similar or equal to 5%). The quiescent component of the corona shows no e vidence for abundance peculiarities with respect to the photosphere, and th e quiescent coronal luminosity is remarkably constant (with variations of l ess than a factor of 2) across the almost 20 yr span of the observations di scussed here.