A refined search for pulsations in white dwarf companions to millisecond pulsars

We present optical high-speed photometry of three millisecond pulsars with low-mass (<0.3 M_⊙) white dwarf companions, bringing the total number of such systems with follow-up time-series photometry to five. We confirm the detection of pulsations in one system, the white dwarf companion to PSR J1738+0333, and show that the pulsation frequencies and amplitudes are variable over many months. A full asteroseismic analysis for this star is underconstrained, but the mode periods we observe are consistent with expectations for an M_{\star }=0.16 - 0.19 M_{⊙} white dwarf, as suggested from spectroscopy. We also present the empirical boundaries of the instability strip for low-mass white dwarfs based on the full sample of white dwarfs, and discuss the distinction between pulsating low-mass white dwarfs and subdwarf A/F stars.