The Carbon Star is 2.6455 ounces lighter than the RS1.
RS1 spec lists 1575 grams or 55.5564 oz. or 3.4 lbs.
Star Carbon lists 1500 grams or 52.9109 or 3.3 lbs

I own a Star Cerwinske (non carbon) and an RS1 in gloss black, both size large. The non carbon Star is 1600 grams, 25 grams heavier than the RS1 - that's a bit less than an ounce. I can't feel any difference in weight between the two. I don't know if the Star spec weight includes the optional stick-on spoiler piece. I reviewed both helmets in detail here on Revzilla but since I own both, for your convenience, a short summary here of how they differ:

The RS1 is "cushier" and quieter. Designed with a little more "all around" comfort in mind, it has a thicker more cushioned liner and is a bit more aerodynamic in upright riding positions. It ventilates well and due to the plusher padding, seals nicely around the neck, which seals out noise and drafts up the neck. Venting inside the helmet is very good.

The Star has a leaner, firmer liner and superb ventilation and ventilation control, which makes it noisier. Airflow through the helmet is fantastic and air does come up the back of the neck. I purchased the thicker, firm cheek pads for the Star to ensure a snug, wiggle free fit. I didn't need to do that for the RS1.

Overall - I think both helmets are fantastic! Honestly, it was the Cerwinske pattern that got me to buy the Star, but once riding with it, it does offer superior airflow and airflow control and the aerodynamics at speed are excellent.

One additional note - PHOTOCHROMIC SHIELD IS AWESOME! If you ride day to night often - I commute and do - its worth every penny. Good luck. Email with questions if you want to know more.