There are light helmets and there's the Catlike Kompact. At 242g it's the lightest XC lid we've tested.
Despite weighing around the same as a wet burp, it passes all relevant safety tests to ensure that your bonce remains intact if you come off. It also offers great head coverage, and 21 large bug-sucking vents result in one of the best aired helmets available.
The Kompact is available in two sizes (MD and LX) and a bunch of colours from the sublime to full Euro roadie - check them out on the Prendas website.