  We have acquired 65 h of single-site time-resolved CCD photometry of the
pulsating DB white dwarf star CBS 114 and 62 h of two-site high-speed CCD
photometry of another DBV, PG 1456+103. The pulsation spectrum of PG 1456+103
is complicated and variable on time scales of about one week and could only
partly be deciphered with our measurements. The modes of CBS 114 are more
stable in time and we were able to arrive at a frequency solution somewhat
affected by aliasing, but still satisfactory, involving seven independent modes
and two combination frequencies. These frequencies also explain the discovery
data of the star, taken 13 years earlier. We find a mean period spacing of 37.1
+/- 0.7 s significant at the 98% level between the independent modes of CBS 114
and argue that they are due to nonradial g-mode pulsations of spherical degree
l=1. We performed a global search for asteroseismological models of CBS 114
using a genetic algorithm, and we examined the susceptibility of the results to
the uncertainties of the observational frequency determinations and mode
identifications (we could not provide m values). The families of possible
solutions are identified correctly even without knowledge of m. Our optimal
model suggests Teff = 21,000 K and M_* = 0.730 M_sun as well as log(M_He/M_*) =
-6.66, X_O = 0.61. This measurement of the central oxygen mass fraction implies
a rate for the ^12C(alpha,gamma)^16O nuclear reaction near S_300=180 keV b,
consistent with laboratory measurements.
