On the Selection of Photometric Planetary Transits

We present a new method for differentiating between planetary transits andeclipsing binaries based on the presence of the ellipsoidal light variations.These variations can be used to detect stellar secondaries with masses ~0.2M_sun orbiting sun-like stars at a photometric accuracy level which has alreadybeen achieved in transit surveys. By removing candidates exhibiting this effectit is possible to greatly reduce the number of objects requiring spectroscopicfollow up with large telescopes. Unlike the usual candidate selection method,which are primarily based on the estimated radius of the orbiting object, thistechnique is not biased against bona-fide planets and brown dwarfs with largeradii, because the amplitude of the effect depends on the transiting object'smass and orbital distance. In many binary systems, where a candidate planetarytransit is actually due to the partial eclipse of two normal stars, thepresence of flux variations due to the gravity darkening effect will show thetrue nature of these systems. We show that many of the recent OGLE-IIIphotometric transit candidates exhibit the presence of significant variationsin their light curves and are likely to be due to stellar secondaries. We findthat the light curves of white dwarf transits will generally not mimic those ofsmall planets because of significant gravitationally induced flux variations.We discuss the relative merits of methods used to detect transit candidateswhich are due to stellar blends rather than planets. We outline how photometricobservations taken in two bands can be used to detect the presence of stellarblends.Comment: ApJ, 11 pages, 2 figures, 1 table, replaced with accepted versio