Interesting rope from Edelrid. Only 25 grams per meter. Static till high load, then dynamic. Very thin 6.5 mm. Nylon/Kevlar/nylon construction. Holds two
falls when used as a twin rope.

http://www.edelrid.de/en/sports/ropes/Accessory-Cords/Rap-Line-II-6-5-mm-pink.html

Mtn gear has them on sale.

Thanks for posting. That is what I call skinny. Anyone have experience with something all of 1/4 " thick. How's the friction? Apparently REI does not carry this model. Why?

I have a set of 7.5mm double/twin ropes, which is already scary thin and as twin the thinnest limit for most regular belay devices. As doubles, I’ll need to grab a different belay device.
I have used Edelrid's flycathers 6.9mm rope. It only comes with a package with the microjul belay device. Scary to belay with such thin rope.

But a 6.5mm as a rap line is actually thickish. Altough the others are not suitable as twins.
- Mammut RAPPEL CORD is 6.0mm
- Petzl RAD LINE is also 6mm

Just beware than these super simple skinny ropes require a special belay device suitable for such a thin rope and rope wear is a bit of an issue.