  An abundance analysis is presented and discussed for a sample of 14 RV Tauri
stars. The present abundance data and those from our previous papers and by
other workers are combined in an attempt to further understanding of the
dust-gas separation process which afflicts many RV Tauri variables. We propose
that a star's intrinsic (i.e., initial) metallicity is given by the
photospheric zinc abundance. Variables warmer that about 5000 K and with an
initial metallicity [Fe/H] $\geq$ $-$1 are affected by dust-gas separation.
Variables of all metallicities and cooler than about $T_{\rm eff} \simeq 5000$
K are unaffected by dust-gas separation. The RV Tauri variables show a spread
in their C abundances with the lower boundary of the points in the C versus Zn
plane falling close to the predicted trend for giants after the first
dredge-up. The upper boundary is inhabited by a few stars that are carbon-rich.
The O abundances in the mean follow the predicted trend from unevolved stars in
line with the expectation that photospheric O abundance is unaffected by the
first dredge-up. An evolutionary scenario involving mass loss by a first ascent
or early-AGB red giant, the primary star of a binary, is sketched.
