Here are all the SRAM X.0 derailleurs that you might need, including both aluminium and carbon cages, both long and short.
The carbon cage version features: Technology: 1:1 actuation ratio & Direct Route Technology for lighter effort shifting. Speeds: 8/9. Weight: 197g Med/192g Short. Material: Forged alloy links and B-knuckle, High strength Grilon P-knuckle, Machined and heat treated oversize pivot pins, sealed cartridge bearing pulleys, carbon fibre cages. Compatibility: SRAM 1:1 shifters (triggers and twisters).