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Black Diamond - Vapor Helmet
(Get this price if shipping outside EU)
Brand: Black Diamond
Black Diamond's Vapor helmet is their addition to the super lightweight category, building on the likes of the Petzl Meteor whilst maintaining a more aesthetic design than the even lighter Petzl Sirocco.
At 186g in it's smallest size it really is lightweight and as Black Diamond themselves hoped it really is so unobtrusive that you find there's no excuse to wearing it for every route.
Having one myself I'm utterly sold, and can't recommend them enough.