  The Kepler spacecraft observed the hot subdwarf star PHL 417 during its
extended K2 mission, and the high-precision photometric lightcurve reveals the
presence of 17 pulsation modes with periods between 38 and 105 minutes. From
follow-up ground-based spectroscopy we find that the object has a relatively
high temperature of 35 600 K, a surface gravity of $\log g / {\rm
cm\,s^{-2}}\,=\,5.75$ and a super-solar helium abundance. Remarkably, it also
shows strong zirconium lines corresponding to an apparent +3.9 dex
overabundance compared with the Sun. These properties clearly identify this
object as the third member of the rare group of pulsating heavy-metal stars,
the V366 Aquarii pulsators. These stars are intriguing in that the pulsations
are inconsistent with the standard models for pulsations in hot subdwarfs,
which predicts that they should display short-period pulsations rather than the
observed longer periods. We perform a stability analysis of the pulsation modes
based on data from two campaigns with K2. The highest amplitude mode is found
to be stable with a period drift, $\dot{P}$, of less than $1.1\cdot10^{-9}$
s/s. This result rules out pulsations driven during the rapid stages of helium
flash ignition.
