  The presence of multiple populations is now well-established in most globular
clusters in the Milky Way. In light of this progress, here we suggest a new
model explaining the origin of the Sandage period-shift and the difference in
mean period of type ab RR Lyrae variables between the two Oosterhoff groups. In
our models, the instability strip in the metal-poor group II clusters, such as
M15, is populated by second generation stars (G2) with enhanced helium and CNO
abundances, while the RR Lyraes in the relatively metal rich group I clusters
like M3 are mostly produced by first generation stars (G1) without these
enhancements. This population shift within the instability strip with
metallicity can create the observed period-shift between the two groups, since
both helium and CNO abundances play a role in increasing the period of RR Lyrae
variables. The presence of more metal-rich clusters having
Oosterhoff-intermediate characteristics, such as NGC 1851, as well as of most
metal-rich clusters having RR Lyraes with longest periods (group III) can also
be reproduced, as more helium-rich third and later generations of stars (G3)
penetrate into the instability strip with further increase in metallicity.
Therefore, for the most general cases, our models predict that the RR Lyraes
are produced mostly by G1, G2, and G3, respectively, for the Oosterhoff groups
I, II, and III.
