  We have observed an unexpected enhancement of the lower critical field
$H_{c1}(T)$ and the critical current $I_{c}(T)$ deep in the superconducting
state below $T \approx 0.6$ K ($T/T_{c} \approx 0.3$) in the filled
skutterudite heavy fermion superconductor PrOs$_4$Sb$_{12}$. From a comparison
of the behavior of $H_{c1}(T)$ with that of the heavy fermion superconductors
U$_{1-x}$Th$_x$Be$_{13}$ and UPt$_3$, we speculate that the enhancement of
$H_{c1}(T)$ and $I_{c}(T)$ in PrOs$_4$Sb$_{12}$ reflects a transition into
another superconducting phase that occurs below $T/T_{c} \approx 0.3$. An
examination of the literature reveals unexplained anomalies in other physical
properties of PrOs$_4$Sb$_{12}$ near $T/T_{c} \approx 0.3$ that correlate with
the features we have observed in $H_{c1}(T)$ and $I_{c}(T)$.
