Petersen Diagram for RRd Stars in the Magellanic Clouds

RRd stars from the Magellanic Clouds form a well-defined band in the Petersen diagram. We explain this observed band with our evolutionary and pulsation calculations with assumed metallicity [Fe/H]=(-2,-1.3). Vast majority of RRd stars from LMC is confined to a narrower range of (-1.7,-1.3). The width of the band, at specified fundamental mode period, may be explained by mass spread at given metallicity. The shape of the band reflects the path of RRd stars within the RR Lyr instability strip. We regard the success in explaining the Petersen diagram as a support for our evolutionary models, which yield mean absolute magnitude in the mid of the instability strip, ⟨MV⟩, in the range 0.4 to 0.65 mag implying distance modulus to the LMC of 18.4 mag.