Forbidden oxygen lines in Comets C/2006 W3 Christensen and C/2007 Q3 Siding Spring at large heliocentric distance: Implications for the sublimation of volatile ices

► We present observations of the atomic oxygen lines in two comets. ► Both comets were 3 AU from the Sun at the time of observation. ► Can use oxygen line observations to constrain CO2 and H2O production. ► We find evidence for enhanced CO2 activity in these comets.