Published online by Cambridge University Press: 25 May 2016
We present the first 12CO 1→0 emission-line maps of the vicinities of two Wolf-Rayet stars (WR 16 and WR 75) and their associated ring nebulae. We illustrate that sizeable amounts of molecular gas appear associated with these ring nebulae and therefore that the mass of gas in ring nebulae is significantly higher than inferred from observations of the ionized gas component alone. We discuss the possible stellar and interstellar origins of these molecular materials and the implications for the evolution of massive stars up to the WR phase.