Relation between normalised tidal potential energy, , and total mass, M_total, of heartbeat systems. Red circles stand for a sample of 14 Kepler HBSs that are red giants (Beck et al. 2014). Green circles are 19 Kepler HBSs with radial-velocity measured by Shporer et al. (2016). Known massive HBSs are marked by dark blue circles and labeled. Our sample of 20 massive HBSs is plotted with light blue circles. The eclipsing systems are encircled; those with detected TEOs are additionally marked with plus signs. See text for further details. The size of symbols depends on the peak-to-peak amplitude of heartbeats; the higher the amplitude, the larger the symbol.