  It is well known that the mere location of a Beat Cepheid model in a Period
Ratio \vs Period diagram (Petersen diagram) puts constraints on its metallicity
Z. But these bounds are sensitive to the mixture of elements that are lumped
into the parameter $Z$. In this short paper we update the previous results that
were based on the Grevesse-Noels solar mixture to the recent, revised Asplund,
Grevesse & Sauval (2005) (AGS) solar mixture.
  We also examine the effect of the envelope depth on the accuracy of the
computed pulsation periods. We find that for low period Cepheids with high Z
the customary approximation of envelope pulsation breaks down. It is necessary
to compute stellar models that extend to the center and to include burning and
composition inhomogeneities in the modeling. Fortunately, however, most Beat
Cepheids that have been observed so far seem to avoid that regime.
