PSR J1738+0333: the first millisecond pulsar + pulsating white dwarf binary.

We report the discovery of the first millisecond pulsar with a pulsating white dwarf (WD) companion. Following the recent discoveries of pulsations in extremely low-mass (ELM, ≤0.3 M_⊙) 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 time-scale with theoretical and empirical observations of pulsations in ≈0.17 M_⊙ 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.