  We use the pulsational properties of the RR Lyrae variables in the globular
cluster NGC 1851 to obtain detailed constraints of the various sub-stellar
populations present along its horizontal branch. On the basis of detailed
synthetic horizontal branch modeling, we find that minor helium variations
(Y~0.248-0.280) are able to reproduce the observed periods and amplitudes of
the RR Lyrae variables, as well as the frequency of fundamental and
first-overtone RR Lyrae stars. Comparison of number ratios amongst the blue and
red horizontal branch components and the two observed subgiant branches also
suggest that the RR Lyrae variables originated from the progeny of the bright
subgiant branch. The RR Lyrae variables with a slightly enhanced helium
(Y~0.270-0.280) have longer periods at a given amplitude, as is seen with
Oosterhoff II (OoII) RR Lyrae variables, whereas the RR Lyrae variables with
Y~0.248-0.270 have shorter periods, exhibiting properties of Oosterhoff I (OoI)
variables. This correlation does suggest that the pulsational properties of RR
Lyrae stars can be very useful for tracing the various sub-populations and can
provide suitable constraints on the multiple population phenomenon. It appears
of great interest to explore whether this conclusion can be generalized to
other globular clusters hosting multiple populations.
