Red novae, stellar mergers in binary and triple systems, and bipolar nebulae

Red novae are transients powered by collisions of non-compact stars. Among their progenitors are systems of evolved subgiants and giants stars. Remnants of such red novae display bipolar structures which have remarkably close characteristics to many post-AGB or pre-PN systems. It is important to ask (and eventually verify) whether some of the less-known post-main-sequence objects (mis-)classified as pre-PNe can be merger remnants similar to the red nova remnants.