For some, cycling caps are too thick and too cottony for use underneath their helmet. For these people, the Campagnolo Sportswear Technical Gear System (TGS) Factory Team bandana is the solution.
Made with ultra-thin, wicking polyester, the "do rag" adds barely a sliver between your noggin and your brain bucket. Campagnolo upped the ante a bit by adding a thin terry cloth headband that prevents sweat from dripping in your eyes.
The team bandana is available in Fire (red), Sun (yellow), and Lake (blue).