  The RR Lyrae instability strip (IS) in NGC 1851 is investigated, and a model
is presented which reproduces the pulsational properties of the RR Lyrae
population. In our model, a stellar component within the IS that displays minor
helium variations (Y~0.248-0.270) is 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 variables. The RR Lyrae variables therefore may
belong to an He-enriched second generation of stars. 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.
