Tight stellar binaries favour active longitudes at sub- and anti-stellar points

ABSTRACT Stellar binaries are ubiquitous in the galaxy and a laboratory for astrophysical effects. We use TESS to study photometric modulations in the light curves of 162 unequal mass eclipsing binaries from the EBLM (Eclipsing Binary Low Mass) survey, comprising F/G/K primaries and M-dwarf secondaries. We detect modulations on 81 eclipsing binaries. We catalogue the rotation rates of the primary star in 69 binaries and discover 17 ellipsoidal variables. In a large portion (at least 51~{\ per\ cent}) of our sample, we detect photometric modulations consistent with two overdensities of spots on the primary star that are roughly 180° apart. We show that these so-called active longitudes are preferentially at the sub- and antistellar points on the primary star. Physically, this means that the spots on the primary star preferentially face directly towards and away from the secondary star.