Model of PSO 318.5-22. Top: PT structures, red is the bright state, blue is the faint state. Middle: spectral flux at a resolution R ~ 25, red is the bright state, blue is the faint state. Bottom: modeled spectral ratio in blue either assuming chemical abundances in the bright state equal to the abundances in the faint state or assuming chemical equlibrium. The observed spectral ratio is in green (see Biller et al. 2018, for details).