Detection of p-mode pulsations and possible ellipsoidal luminosity variations in the hot subdwarf B star KPD 1930+2752

We report the discovery of multiperiodic luminosity variations in the hot B subdwarf KPD 1930 + 2752. This star was selected as a potential target in the course of our ongoing survey to search for pulsators of the EC 14026 type. Our model atmosphere analysis of the time-averaged optical spectrum of KPD 1930 + 2752 indicates that this star has T-eff similar or equal to 33,280 K and log g similar or equal to 5.61, which places it well within the theoretical EC 14026 instability strip. At least 44 harmonic oscillations are seen in the light curve, with periods in the range 145-332 s, and amplitudes in the range 0.064%-0.451% of the mean brightness of the star. In addition, the light curve is dominated by a nearly sinusoidal variation with a period of similar to 4108.9 s and amplitude of similar to 1.39%. This latter variation is unique among the known EC 14026 stars. We argue that this relatively slow luminosity variation is likely due to the ellipsoidal deformation of the sdB star in a close binary system containing a faint invisible companion (possibly a white dwarf). Using a new period-matching technique based on a genetic algorithm, we demonstrate that the dense observed period spectrum in the 145-332 s interval is compatible with a theoretical low-degree p-mode spectrum that is rotationally split in a star rotating with a period of similar to 8217.8 s, the value expected from the ellipsoidal effect invoked to explain the observed long-period variation. This interpretation awaits the test of time-resolved spectroscopy. If confirmed, the potential of KPD 1930 + 2752 as a laboratory for EC 14026 seismology will become immense.