  We present a grid of evolutionary tracks for low-mass white dwarfs with
helium cores in the mass range from 0.179 to 0.414 M_sun. The lower mass limit
is well suited for comparison with white dwarf companions of millisecond
pulsars (MSP). The derived cooling ages are of the order of 10^9 yrs due to
residual nuclear burning. The cooling ages are consistent with age estimations
of MSP systems based on the pulsars' spin-down. For example, for the system PSR
1012+5307 we derived a white dwarf cooling age of 6 +/-1 Gyr in good agreement
with the spin-down age of 7 Gyr. For the companion mass we found M=0.19 +/-
0.02 M_sun. We studied other MSP systems as well selecting only systems with
well given ages and/or masses, and determined the effective temperatures and
surface gravities of the companion white dwarfs with the present evolutionary
models.
