WZ Sge: an eclipsing cataclysmic variable evolving towards the period minimum

Abstract

We present the photometric results of the eclipsing cataclysmic variable (CV) WZ Sge near the period minimum ($P_{min}$). Eight new mid-eclipse times were determined and the orbital ephemeris was updated. Our result shows that the orbital period of WZ Sge is decreasing at a rate of $\dot{P}=-2.72(\pm0.23)\times{10^{-13}}\,s s^{-1}$. This secular decrease, coupled with previous detection of its donor, suggest that WZ Sge is a pre-bounce system. Further analysis indicates that the observed period decrease rate is about $1.53$ times higher than pure gravitational radiation (GR) driving. We constructed the evolutionary track of WZ Sge, which predicts that $P_{min}$ of WZ Sge is $\sim77.98 (\pm0.90)$ min. If the orbital period decreases at the current rate, WZ Sge will evolve past its $P_{min}$ after $\sim25.3$ Myr. Based on the period evolution equation we find $\dot{M}_{2}\simeq4.04(\pm0.10)\times10^{-11}M_{\odot}yr^{-1}$, which is compatible with the current concept of CV evolution at ultrashort orbital periods.