The Period Changes of the Cepheid RT Aurigae

Observations of the light curve for the 3.7 day Cepheid RT Aur both before and since 1980 indicate that the variable is undergoing an overall period increase, amounting to +0.082 +/- 0.012 s yr^-1, rather than a period decrease, as implied by all observations prior to 1980. Superposed on the star's O-C variations is a sinusoidal trend that cannot be attributed to random fluctuations in pulsation period. Rather, it appears to arise from light travel time effects in a binary system. The derived orbital period for the system is P=26,429+/-89 days (72.36+/-0.24 yr). The inferred orbital parameters from the O-C residuals differ from those indicated by existing radial velocity data. The latter imply the most reasonable results, namely a₁sini=9.09(+/-1.81)×10⁸ km and a minimum secondary mass of M₂=1.15+/-0.25 M_solar. Continued monitoring of the brightness and radial velocity changes in the Cepheid are necessary to confirm the long-term trend and to provide data for a proper spectroscopic solution to the orbit.