Dependence of pulsation mode of Cepheids on metallicity
  The Cepheid variables in SMC, LMC, the Milky Way, M33 and M31 are used to
examine the dependence of pulsation mode on metallicity which was previously
found in red supergiants. The initial samples of Cepheids are collected from
the Cepheid catalogs identified from the OGLE, PS1, DIRECT, WISE and ZTF
surveys. The contaminants are removed with the help of the Gaia/EDR3
astrometric information for extra galaxies or by comparing the geometric
distance and the distance from the P-L relation for the Milky Way. The division
of fundamental and first-overtone mode is refined according to the gap between
the two modes in the P-L diagram of the objects in each galaxy. The ratio of
FU/(FU+1O) is found to be 0.59, 0.60, 0.69, 0.83 and 0.85 for SMC, LMC, the
Milky Way, M33 and M31 respectively in order of metallicity, which confirms
that the pulsation mode depends on metallicity in the way that the ratio of
FU/(FU+1O) increases with metallicity. This dependence is not changed if the
incompleteness of the samples is taken into account.
