Discovery of a new class of pulsating stars: Gravity-mode pulsators among subdwarf B stars

During the course of an ongoing CCD monitoring program to investigate low-level light variations in subdwarf B (sdB) stars, we serendipitously discovered a new class of multimode pulsators with periods of the order of an hour. These periods are a factor of ten longer than those of previously known multimode sdB pulsators (EC 14026 stars), implying the new pulsations are due to gravity modes rather than pressure modes. The iron opacity instability that drives the short period EC 14026 stars is effective in hot sdB's. The long period pulsators are found only among cooler sdB stars, where they are surprisingly common. The mechanism responsible for exciting the deeper g-modes in cool sdB's is currently unknown, but the temperature and gravity range in which these stars occur must be an important clue. We present the first observational results for this new class of pulsating sdB stars, and discuss some possible implications.