the least abundant of these two elements will likely remain in the gas phase as COx. In the dust formation zone, refractory elements and compounds (Fe, Si, MgO, etc.) are removed from the gas phase and end up in dust grains. The newly formed dust will immediately assist in surface catalyzed reactions. The stellar winds from AGB stars are sites of cosmic dust formation, and are believed to be the main production sites of dust in the universe. The stellar winds of AGB stars (Mira variables and OH/IR stars) are also often the site of maser emission. The molecules that account for this are SiO, H2O, OH, HCN, and SiS. SiO, H2O, and OH masers are typically found in oxygen-rich M-type AGB stars such as R Cassiopeiae and U Orionis, while HCN and SiS masers are generally found in carbon stars such as IRC +10216. S-type stars with masers are uncommon. After these stars have lost nearly all of their envelopes, and only the core regions remain, they evolve further into short-lived protoplanetary nebula. The final fate of the AGB envelopes are represented by planetary nebulae (PNe). Late thermal pulse As many as a quarter of all post-AGB stars undergo what is dubbed a "born-again" episode. The carbon–oxygen core is now surrounded by helium with an outer shell of hydrogen. If the helium is re-ignited a thermal pulse occurs and the star quickly returns to the AGB, becoming a helium-burning, hydrogen-deficient stellar object. If the star still has a hydrogen-burning shell when this thermal pulse occurs, it is termed a "late thermal pulse". Otherwise it is called a "very late thermal pulse". The outer atmosphere of the born-again star develops a stellar wind and the star once more follows an evolutionary track across the Hertzsprung–Russell diagram. However, this phase is very brief, lasting only about 200 years before the star again heads toward the white dwarf stage. Observationally, this late thermal pulse phase appears almost identical to a Wolf–Rayet star in the midst of its own planetary nebula. Stars such as Sakurai's Object and FG Sagittae are being observed as they rapidly evolve through this phase. Mapping the circumstellar magnetic fields of thermal-pulsating (TP-) AGB stars has recently been reported using the so called Goldreich-Kylafis effect. Super-AGB stars Stars close to the upper mass limit to still qualify as AGB stars show some peculiar properties and have been dubbed super-AGB