The effect of star-spots on eclipse timings of binary stars

We investigate the effects that star-spots have on the light curves of eclipsing binaries, and in particular how they may affect the accurate measurement of eclipse timings. Concentrating on systems containing a low-mass main-sequence star and a white dwarf, we find that if star-spots exhibit the Wilson depression they can alter the times of primary eclipse ingress and egress by several seconds for typical binary parameters and star-spot depressions. In addition, we find that the effect on the eclipse ingress/egress times becomes more profound for lower orbital inclinations. We show how it is possible, in principle, to determine estimates of both the binary inclination and the depth of the Wilson depression from light curve analysis The effect of depressed star-spots on the O−C diagrams of eclipsing systems is also investigated. It is found that the presence of star-spots will introduce a jitter in the O−C residuals and can cause spurious orbital period changes to be observed. Despite this, we show that the period can still be accurately determined even for heavily spotted systems.