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Session 45 - Eclipsing Binaries.
Display session, Thursday, January 08
VW Cep is one of the brightest and longest observed short-period (P= 6.67 hours) W UMa type binaries. We investigated complex period changes based upon 880 eclipse timings from the past 70 years, including 12 obtained in the Summer of 1997 at Eastern College Observatory. In addition to the well-known 30.9 year light time effect due to the presence of a third star in the system, we find evidence for a long term decrease in the orbital period of dP/dt = -0.018 sec/yr. This decrease in period could arise from angular momentum loss from the binary and/or mass exchange between components. The dominant change in O-Cís over the past 70 years seems to imply a rather constant change in period because it can be fit reasonably well with a parabolic fit.
We subtracted out the best fit parabola which then presumably left us with only the 3rd body light-time effects. By determining the best fit to these residuals we refined the properties of the tertiary component. The light-time corrections were then subtracted, leaving us small systematic deviations in the residual O-C's. These second order variations appear to be dynamical in nature and not due to the presence of a fourth body since the period of the variation has not remained constant over the 70 year observation interval. The fluctuations over the last 15 years have a period of approximately 5.8 years with a semiamplitude of 0.004 days.
The changes in the light curve and luminosity of the system most likely arise from growth and decay of magnetically induced starspots. If this is the case, then the 5.8 year cycle found in the O-C residuals could be in response to a magnetic activity cycle operating in the system. This research is supported by an NSF/RUI Grant AST93-15365 which we gratefully acknowledge.
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