  We present the analysis of 71 hours of high quality time-series CCD
photometry of the helium atmosphere pulsating white dwarf (DBV) EC 04207-4748
obtained using the facilities at Mt John University Observatory in New Zealand.
The photometric data set consists of four week-long observing sessions covering
the period March to November 2011. A Fourier analysis of the lightcurves
yielded clear evidence of four independent eigenmodes in the star with the
dominant mode having a period of 447 s. The lightcurve variations exhibit
distinct nonsinusoidal shapes, which results in significant harmonics of the
dominant frequency appearing in the Fourier transforms. These observed
variations are interpreted in terms of nonlinear contributions from the energy
flux transmission through the subsurface convection zone in the star. Our
modelling of this mechanism, using the methods first introduced by Montgomery
(2005), yields a time-averaged convective response time of tau_0 ~ 150 s for
the star, and this is shown to be broadly consistent with a MLT/alpha parameter
value between 0.8 and 1.2. It is argued that for the DBV pulsators the measured
value of tau_0 is a better estimate of the relative stellar surface
temperatures than those obtained via spectroscopic techniques.
