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J.R. Hargis, D.H. Bradstreet, J.S. Lake, F.D. Jewett, G.R. Wilkinson, J.M. Blair (Eastern College), E.F. Guinan, I. Ribas, G.P. McCook (Villanova University)
We present the analysis of 85 times of minimum light of the eclipsing binary CM Draconis (dM4.5+dM4.5, P=1.268 days, Vmax=11.5 mag) obtained between 1995 and mid-2000. With the possibility of planetary transits in this system (see IAUC 6423), we have been carrying out I-band photometry with particular attention being paid to observing the primary and secondary eclipses. In addition to searching for transit events, we have been monitoring the eclipse minima to look for evidence of the ``light-time'' effect that arises from the gravitational influence of a tertiary companion. The presence of a third body causes the relative distance of the eclipsing pair to the Earth to change as it orbits the barycenter of the triple system.
The observed-computed (O-C) timings (from a linear ephemeris) determined from the analysis of the eclipse minima show evidence of small (~0-20 s) variations on different time scales. The magnitude of the short-term variations appears to be correlated with the presence of wave-like disturbances in the outside eclipse portions of the light curves. These quasi-sinusoidal light variations are attributed to the presence of star spots on these two rapidly-rotating dM4.5 stars. Here we show that the observed time of minimum light can be displaced typically by ~0-15 seconds due to an uneven distribution of star spots on the surface of the star undergoing the eclipse. The preliminary results indicate that star spots can indeed account for the short-term, non-periodic variations in the timing residuals. Models of CM Dra generated in Binary Maker with different spot coverages and distributions will be presented showing their effects on the time of minimum light determination. Modeling of the eclipses and eclipse times offer an opportunity to map the spots, as well their temporal evolution, on the surfaces of the two stars.
Although the short-term variations in the O-Cs arise most likely from spots, there appears to be a long-term variation in the seasonal averages with a period of several years. The semi-amplitude of this variation is about 10 seconds. If future observations confirm that this long-term variation in the O-Cs is indeed periodic, then it could be evidence for the presence of a sub-stellar third body in an eccentric orbit around the eclipsing binary.
This research is partially supported by NSF/RUI grants AST93-15365 and AST00-71260.