Further tests show that if you disable the brown-out detection (by changing a fuse byte) you can greatly save power, especially in sleep mode. With brown-out detection disabled power consumption goes down to around 6 uA. This is because the brown-out detection needs an internal voltage reference to be running.
T1 conducts when the optocoupler sends a pulse to its base. C1 discharges through the collector-emitter line. The trigger current is limited by R2 to around 40 mA. The discharge current time of C1 is less than 1 ms.