During the course of an ongoing CCD monitoring program to investigate low-level light variations in subdwarf B (sdB) stars, we have serendipitously discovered a new class of multimode sdB pulsators with periods on the order of an hour. The approximate peak-to-peak amplitudes are less than a few hundredths of a magnitude. Although a good multi-site campaign would be required for better mode characterization, a first analysis of 81 hours monitoring of the prototype, PG1716+426, over 14 nights (spread out over 3 months) shows the presence of 5 modes with periods between 0.8 and 1.4 hours. We have so far identified more than a dozen new long period sdB pulsators, all falling in the range 25,000 < Teff < 30,000 K and 5.4 < log g < 5.7.
The newly identified pulsation periods are a factor of 10 longer than typical periods of previously known nonradial multimode sdB pulsators, i.e. the EC 14026 stars (see, e.g., Koen et al. 1999, MNRAS, 306, 213; Charpinet, Fontaine, & Brassard 2001, PASP, 113, 775). The pulsations in EC 14026 stars are due to acoustic modes (p-modes), an envelope phenomenon caused by an iron opacity instability that is predicted and observed to be largest in the hottest sdB stars (Teff 34,000 K). The longer periods now being detected in cooler sdB stars imply the presence of excited high radial order gravity modes (g-modes). The mechanism able to excite such core modes in sdB stars is currently unknown. Nevertheless, the relatively long pulsation timescales, the distinctly different temperature range, and the inability of the iron opacity mechanism to drive g-modes, all support the identification of the long period sdB's as a separate class of pulsating stars. This work has been supported by NSF grants AST-9731655 and AST-0098699.

 
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