KIC 2856960: The impossible triple star

Abstract

KIC 2856960 is a star in the Kepler field which was observed by Kepler for four years. It shows the primary and secondary eclipses of a close binary of period 0.258 d as well as complex dipping events that last for about 1.5 d at a time and recur on a 204 d period. The dips are thought to result when the close binary passes across the face of a third star. In this paper, we present an attempt to model the dips. Despite the apparent simplicity of the system and strenuous efforts to find a solution, we find that we cannot match the dips with a triple star while satisfying Kepler's laws. The problem is that to match the dips, the separation of the close binary has to be larger than possible relative to the outer orbit given the orbital periods. Quadruple star models can get round this problem but require the addition of a so-far undetected intermediate period of the order of 5-20 d that has been a near-perfect integer divisor of the outer 204 d period. Although we have no good explanation for KIC 2856960, using the full set of Kepler data we are able to update several of its parameters.We also present a spectrum showing that KIC 2856960 is dominated by light from a K3- or K4-type star.