  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 Lyrae instability strip. We regard the success in explaining the
Petersen diagram a support for our evolutionary models, which yield, mean
absolute magnitude in the mid of the instability strip, <M_V>, in the range 0.4
to 0.65 mag implying distance modulus to LMC of 18.4 mag.
