Detection of p-mode pulsations in the hot subdwarf b star KPD 2109+4401

We report the discovery of multiperiodic luminosity variations in the hot B subdwarf KPD 2109+4401. At least five periodicities are seen in the light curve, from 182.5 to 198.4 s. The largest oscillation has an amplitude similar or equal to 8.6 mmag in white light and a period 196.3 s. Our model atmosphere analysis of the time-averaged optical spectrum of KPD 2109+4401 indicates that this star has T-eff similar or equal to 31,200 K and log g similar or equal to 5.84. A comparison with pulsation periods computed from stellar models having similar atmospheric parameters implies that the observed brightness variations must be identified with low-order radial and nonradial (p and f) pulsation modes. The overall similarity of periods and derived stellar parameters shows that KPD 2109+4401 is a genuine member of the EC 14026 class, the most recent family of pulsators uncovered in the field of asteroseismology. We note, however, that KPD 2109+4401 is slightly but significantly cooler than the previously known members of this class, thus widening significantly the empirical instability strip. Interestingly, theory predicts that a cooler EC 14026 pulsator (of a given surface gravity) should show larger excited periods, and this is indeed what KPD 2109+4401 shows, with periods reaching values as large as 198 a, not seen previously in other pulsators of the class. We also report on the current status of our ongoing survey to search for additional EC 14026 pulsators.