  [Context.] The hot subdwarf B star HS0702+6043 is known as a large-amplitude,
short-period p-mode pulsator of the EC14026 type. Its atmospheric parameters
place it at the common boundary between the empirical instability regions of
the EC14026 variables and the typically cooler long-period g-mode pulsators of
the PG1716 kind.
  [Aims.] We analyse and interpret the photometric variability of HS0702+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 ~1h in
HS0702+6043, in addition to the two previously known pulsations at 363s and
383s. In particular, we isolate a relatively low-amplitude (~4 mmag),
long-period (3538+-130s) 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, HS0702+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 HS0702+6043 thus
becomes a target of choice for future investigations. While our discovery
appears consistent with the location of HS0702+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.
