  The double superconducting transition in PrOs$_4$Sb$_{12}$, first observed in
specific heat $C(T)$ measurements of single crystal samples, is the subject of
an in-depth study. The double superconducting transitions of a batch of single
crystals were measured before and after they were annealed for 5 days at
$500^{\circ}$C, with no observed change. $C(T)$ measurements near $T_c$ for
PrOs$_4$Sb$_{12}$ in several magnetic fields are also presented, detailing the
evolution of the double transition up to 1 T. Samples of
Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$ with $0.01 \leq x \leq 0.04$ also appear to
display a double superconducting transition in specific heat. In addition,
samples with the smallest Ru concentration measured ($x=0.01$) may even display
a different type of superconductivity than pure PrOs$_4$Sb$_{12}$ ($x=0$).
