  We present optical high-speed photometry of three millisecond pulsars with
low-mass ($< 0.3 M_{\odot}$) 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
under-constrained, but the mode periods we observe are consistent with
expectations for a $M_{\star} = 0.16 - 0.19 M_{\odot}$ 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.
