Context :

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 } \sim 70 L _ { \odot } and \dot { M } \sim 1.1 \times 10 ^ { -8 } M _ { \odot } 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 } = \dot { M } _ { pre - OB } \cdot \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 \times 10 ^ { -7 } M _ { \odot } ) for M _ { 1 } \sim 1.37 M _ { \odot } , 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 _ { \odot } and log \dot { 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 _ { \odot } .
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 .

Conclusions :