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 \sim 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 \sim 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 \sim 0.248-0.270 have shorter periods , exhibiting properties of Oosterhoff I ( OoI ) variables .