  We present metallicities, [Fe/H], and elemental abundance ratios, [X/Fe], for
a sample of 24 Cepheids in the outer Galactic disk. The sample have
Galactocentric distances covering 12 < R_GC (kpc) < 17.2 making them the most
distant Galactic Cepheids upon which detailed abundance analyses have been
performed. We find sub-solar ratios of [Fe/H] and overabundances of [alpha/Fe],
[La/Fe], and [Eu/Fe]. All abundance ratios exhibit a dispersion that exceeds
the measurement uncertainties. As seen in our previous studies of old open
clusters and field giants, enhanced ratios of [alpha/Fe] and [Eu/Fe] reveal
that recent star formation has taken place in the outer disk.
  The outer disk Cepheids appear to exhibit a bimodal distribution for [Fe/H]
and [alpha/Fe]. Most of the Cepheids continue the trends with Galactocentric
distance exhibited by Andrievsky's larger Cepheid sample and we refer to these
stars as the "Galactic Cepheids". A minority of the Cepheids show considerably
lower [Fe/H] and higher [alpha/Fe] and we refer to these stars as the "Merger
Cepheids". One signature of a merger event would be composition differences
between the "Galactic" and "Merger" Cepheids. The Cepheids satisfy this
requirement and we speculate that the distinct compositions suggest that the
"Merger Cepheids" may have formed under the influence of significant merger or
accretion events. The short lifetimes of the Cepheids reveal that the merger
event may be on-going with the Monoceros ring and Canis Major galaxy being
possible merger candidates. [Abridged]
