Abundance analyses of field RV Tauri stars. VI. An extended sample

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 that 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 than about 5000 K and with an initial metallicity [Fe/H] >= -1 are affected by dust-gas separation. Variables of all metallicities and cooler than about T-eff similar or equal to 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.