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A Study of the Orbital Periods of Deeply Eclipsing SW Sextantis Stars

Results are presented of a five-year project to study the orbital periods of eighteen deeply eclipsing novalike cataclysmic variables, collectively known as SW Sextantis stars, by combining new measurements of eclipse times with published measurements stretching back in some cases over fifty years. While the behaviour of many of these binary systems is consistent with a constant orbital period, it is evident that in several cases this is not true. Although the time span of these observations is relatively short, evidence is emerging that the orbital periods of some of these stars show cyclical variation with periods in the range 10-40 years. The two stars with the longest orbital periods, V363 Aur and BT Mon, also show secular period reduction with rates of -6.6 \times 10^{-8} days/year and -3.3 \times 10^{-8} days/year. New ephemerides are provided for all eighteen stars to facilitate observation of future eclipses.