  We report observations of a two-stage coronal dimming in an eruptive event of
a two-ribbon flare and a fast coronal mass ejection (CME). Weak gradual dimming
persists for more than half an hour before the onset of the two-ribbon flare
and the fast rise of the CME. It is followed by abrupt rapid dimming. The
two-stage dimming occurs in a pair of conjugate dimming regions adjacent to the
two flare ribbons, and the flare onset marks the transition between the two
stages of dimming. At the onset of the two-ribbon flare, transient brightenings
are also observed inside the dimming regions, before rapid dimming occurs at
the same places. These observations suggest that the CME structure, most
probably anchored at the twin dimming regions, undergoes a slow rise before the
flare onset, and its kinematic evolution has significantly changed at the onset
of flare reconnection. We explore diagnostics of the CME evolution in the early
phase with analysis of the gradual dimming signatures prior to the CME
eruption.
