  Thanks to their continuous cooling and relative simplicity, white dwarf stars
are routinely used to measure the ages of stellar populations. The usefulness
of white dwarfs as cosmochronometers depends on the availability of accurate
cooling models. A key ingredient of those models are the conductive opacities,
which largely govern the cooling rate. In this work, we present improved
conductive opacities for the regime of moderate coupling and moderate
degeneracy that characterizes an important portion of the envelopes of DA and
DB white dwarfs. We find differences of up to a factor 3 between our
calculations and the commonly used opacities of Cassisi et al. (2007), which we
attribute to an improved account of electron-electron scattering. The cooling
models are strongly affected by those changes in the conductive opacities: the
age of a 4000 K white dwarf can be reduced by as much as 2 Gyr. We provide
analytical fits to our new opacities to facilitate the implementation of this
important effect in white dwarf evolution codes.
