  We report the discovery of the first millisecond pulsar with a pulsating
white dwarf companion. Following the recent discoveries of pulsations in
extremely low-mass (ELM, <0.3 Msol) white dwarfs (WDs), we targeted ELM WD
companions to two millisecond pulsars with high-speed Gemini photometry. We
find significant optical variability in PSR J1738+0333 with periods between
roughly 1790-3060 s, consistent in timescale with theoretical and empirical
observations of pulsations in 0.17 Msol He-core ELM WDs. We additionally put
stringent limits on a lack of variability in PSR J1909-3744, showing this ELM
WD is not variable to <0.1 per cent amplitude. Thanks to the accurate distance
and radius estimates from radio timing measurements, PSR J1738+0333 becomes a
benchmark for low-mass, pulsating WDs. Future, more extensive time-series
photometry of this system offers an unprecedented opportunity to constrain the
physical parameters (including the cooling age) and interior structure of this
ELM WD, and in turn, the mass and spin-down age of its pulsar companion.
