HS 0702+6043: a star showing both short-period p-mode and long-period g-mode oscillations

Context. The hot subdwarf B star HS 0702+6043 is known as a large-amplitude, short-period p -mode pulsator of the EC 14026 type. Its atmospheric parameters place it at the common boundary between the empirical instability regions of the EC 14026 variables and the typically cooler long-period g -mode pulsators of the PG 1716 kind. Aims. We analyse and interpret the photometric variability of HS 0702+6043 in order to explore its asteroseismological potential. Methods. We report on rapid wide band CCD photometric observations to follow up on and confirm the serendipitous discovery of multiperiodic long-period luminosity variations with typical time scales of ~1 h in HS 0702+6043, in addition to the two previously known pulsations at 363 s and 383 s. In particular, we isolate a relatively low-amplitude (~4 mmag), long-period ($3538\pm130$ s) light variation. Results. We argue that the most likely origin for this luminosity variation is the presence of an excited g -mode pulsation. If confirmed, HS 0702+6043 would constitute a rare addition to the very select class of pulsating stars showing simultaneously parts of their pressure and gravity mode pulsation spectra. The asteroseismological potential of such stars is immense, and HS 0702+6043 thus becomes a target of choice for future investigations. While our discovery appears consistent with the location of HS 0702+6043 at the common boundary between the two families of pulsating sdB stars, it does challenge theory's current description of stability and driving mechanisms in pulsating B subdwarfs. Conclusions.