The orbital period and system parameters of the recurrent nova TPyx

TPyx 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 h. We here provide the first spectroscopic confirmation of the orbital period, P= 1.8295h (f= 13.118368 ±1.1 × 10^-5cd^-1), based on time-resolved optical spectroscopy obtained at the Very Large Telescope and the Magellan telescope. We also derive an upper limit of the velocity semi amplitude of the white dwarf, K1 = 17.9 ±1.6kms^-1, and estimate a mass ratio of q= 0.20 ±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 M₂= 0.14 ±0.03 M. This implies a mass of the primary white dwarf of M₁= 0.7 ±0.2 M. If the white dwarf mass is >1M, as classical nova models imply, the donor mass must be even higher. We therefore rule out the possibility that TPyx has evolved beyond the period minimum for cataclysmic variables. We find that the system inclination is constrained to be i≈ 10°, confirming the expectation that TPyx is a low inclination system. We also discuss some of the evolutionary implications of the emerging physical picture of TPyx. 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.