Just caught sight of the new Terra Nova Moonlite Bag cover. Interesting. The information given by TN is limited but it appears to be a fully waterproof, breathable bag cover with taped seams that comes in at just over 200g! It's not a full, zipped bivvy but rather a bag-cover with drawstring, similar to the Alpkit Hunka and RAB survival zone. However, TN claim a weight far lower than either the Hunka or the Survival Zone (Moonlite 216g, Hunka variously reported to be around 400g and the Survival zone also around 400g). It seems to be a good option for use with a tarp in wet climes and would save me 160g against my MLD event Alpine bivvy (although the latter is a fully functional stand-alone bivvy). The weight of the MLD bag is so low that it was never worth my while consideing bag covers (no weight advantage and a serious trade-off in functionality). However, the Terra Nova product might shift the balance. I'd like to know some more about construction and materials though. TN aren't very forthcoming.