The Enigmatic Light Curve of the Eclipsing Binary FF Aqr and the Nature of its Hot Star

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We present CCD (B band) eclipse observations of FF Aqr (K0III + sdOB) that were obtained during the 2005 and 2006 seasons using the SARA 0.9m at KPNO. Available red and UV spectra were used to measure velocities of each component and a mass ratio was determined (q=4.8). The UV spectra also allowed us to determine the effective temperature and surface gravity of the hot subdwarf component and, for the first time, conduct an abundance analysis of its atmosphere. PHOEBE was used to refine the binary's parameters and model the light and velocity curves.
Two different variations are evident in the light curves. One is a gradual dimming of the entire curve ( 0.2mag) that appears to bottom out in 2006. This is likely due to the migrating photometric wave caused by spots on the giant star. There are also other small amplitude ( 0.015mag) variations that exhibit short periods ( 5 mins and 1 hr). This variability shows up both during and after total eclipse of the hot star, which suggests that the red giant is the source. We consider flickering, like that seen by HST in the red giants of the Ursa Minor dwarf spheroidal galaxy, as a possible mechanism.
This work was supported in part by NSF grant AST-0206115 (T.D. Oswalt Principal Investigator) to Florida Institute of Technology. T.V. would like to thank Francis Marion University for travel support.

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