  We use baryon acoustic peak measurements by Eisenstein et al. (2005) and
Percival et al. (2007a) and galaxy cluster gas mass fraction measurements of
Allen et al. (2008) to constrain parameters of three different dark energy
models. For time-independent dark energy, the Percival et al. (2007a)
constraints, which make use of the WMAP measurement of the apparent acoustic
horizon angle, most effectively constrain a cosmological parameter close to
spatial curvature and favor a close to spatially flat model. In a
spatially-flat model the Percival et al. (2007a) data less effectively
constrain time-varying dark energy. The joint baryon acoustic peak and galaxy
cluster gas mass constraints are consistent with but tighter than those derived
from other data. A time-independent cosmological constant in a spatially-flat
model provides a good fit to the joint data, but slowly-evolving dark energy
can not yet be ruled out.
