OH-selected AGB and post-AGB objects. I.

The usual mid-IR selection criteria for post-AGB, based on IRAS colors, largely fail to distinguish early post-AGB stages. A two-color diagram from much narrower band MSX flux densities, with bimodal distributions, provides a better tool for doing this.

Four mutually consistent selection criteria for OH-masing red protoplanetary nebulae are given, as well as two for early post-AGB masers and one for all post-AGB masers including the earliest ones. All these criteria miss a group of blue, high-outflow post-AGB sources with 60{mu}m excess; these will be discussed in detail in Paper II (Sevenster, 2002AJ....123.2772S). The majority of post-AGB sources show regular double-peaked spectra in the OH 1612MHz line, with fairly low outflow velocities, although the fractions of single peaks and irregular spectra may vary with age and mass.