T Pyx is a luminous recurrent nova that accretes at a much higher rate than is expected for its photometrically determined orbital period of about 1.8 hours .
We here provide the first spectroscopic confirmation of the orbital period , P = 1.8295 hours ( f = 13.118368 \pm 1.1 \times 10 ^ { -5 } c/d ) , based on time-resolved optical spectroscopy obtained at the VLT and the Magellan telescopes .
We also derive an upper limit of the velocity semi-amplitude of the white dwarf , K 1 = 17.9 \pm 1.6 km s ^ { -1 } , and estimate a mass ratio of q = 0.20 \pm 0.03 .
If the mass of the donor star is estimated using the period-density relation and theoretical main-sequence mass-radius relation for a slightly inflated donor star , we find M _ { 2 } = 0.14 \pm 0.03 M _ { \odot } .
This implies a mass of the primary white dwarf of M _ { 1 } = 0.7 \pm 0.2 M _ { \odot } .
If the white-dwarf mass is > 1 M _ { \odot } , as classical nova models imply , the donor mass must be even higher .
We therefore rule out the possibility that T Pyx has evolved beyond the period minimum for cataclysmic variables .
We find that the system inclination is constrained to be i \approx 10 degrees , confirming the expectation that T Pyx is a low-inclination system .
We also discuss some of the evolutionary implications of the emerging physical picture of T Pyx .
In particular , we show that epochs of enhanced mass transfer ( like the present ) may accelerate or even dominate the overall evolution of the system , even if they are relatively short-lived .
We also point out that such phases may be relevant to the evolution of cataclysmic variables more generally .