ON THE EXPECTED ORBITAL PERIOD CHANGE IN THE NEXT OUTBURST OF THE RECURRENT NOVA T-PYXIDIS

In view of the fact that the recurrent nova T Pyx is expected to undergo an outburst in the very near future, I calculate the theoretically expected orbital period change resulting from an outburst. It is shown that the period change is a consequence of two, somewhat uncertain, competing effects. A calculation based on our present understanding of the common envelope phase that follows nova explosions, using two methods, predicts a separation decrease of order DELTA-alpha/alpha approximately -5 x 10(-6). Observations aimed at determining the period change will provide us with invaluable information on the physics of novae. I thus strongly urge observers to determine the preoutburst orbital period reliably, to an accuracy of 10(-6).