The secrets of T Pyxidis II. A recurrent nova that will not become a SN Ia

Aims. We compare the observed and theoretical parameters for the quiescent and outburst phases of the recurring nova T Pyx. Methods. IUE data were used to derive the disk luminosity and the mass accretion rate, and to exclude the presence of quasi-steady burning at the WD surface. XMM-NEWTON data were used to verify this conclusion. Results. By various methods, we obtained L-disk similar to 70 L-circle dot and M similar to 1.1 x 10(-8) M-circle dot yr(-1). These values were about twice as high in the pre-1966-outburst epoch. This allowed the first direct estimate of the total mass accreted before outburst, M-accr = Mpre-OB .Delta t, and its comparison with the critical ignition mass M-ign. We found M-accr and M-ign to be in perfect agreement (with a value close to 5 x 10(-7) M-circle dot) for M-1 similar to 1.37 M-circle dot, which provides a confirmation of the thermonuclear runaway theory. The comparison of the observed parameters of the eruption phase, with the corresponding values in the grid of models by Yaron and collaborators, provides satisfactory agreement for values of M-1 close to 1.35 M-circle dot and log M between -8.0 and -7.0, but the observed value of the decay time t(3) is higher than expected. The long duration of the optically thick phase during the recorded outbursts of T Pyx, a spectroscopic behavior typical of classical novae, and the persistence of P Cyg profiles, constrains the ejected mass M-ign to within 10(-5)-10(-4) M-circle dot. Therefore, T Pyx ejects far more material than it has accreted, and the mass of the white dwarf will not increase to the Chandrasekhar limit as generally believed in recurrent novae. A detailed study based on the UV data excludes the possibility that T Pyx belongs to the class of the supersoft X-ray sources, as has been postulated. XMM-NEWTON observations have revealed a weak, hard source and confirmed this interpretation.