I think the key word here is “lightweight,” Aurora. To me, that suggests a down bag. In the 10- to 15-degree temperature range, down will have a better chance of surviving the trip, and you’ll save a pound or more over a synthetic-fill bag.
You have solid choices. One is the Marmot Helium ($389). It’s rated to 15 degrees, has fluffy 850-fill down, and weighs just two pounds, two ounces. That’s pretty impressive, and it compresses to about the size of a cantaloupe.
I also really, really like Western Mountaineering’s Apache MF bag ($410). It’s just a touch lighter than the Helium, rated to 15 degrees, and features Western’s exceptional construction.
Another good bag is Montbell’s U.L. Super Spiral Down Hugger #1 ($399). It weighs two pounds, four ounces is made out of a stretchy shell that gently moulds to your body, which helps keep out cold air when you move around. I’ve tried it, and it works.
Two more suggestions: I highly recommend taking a silk bag liner, one like Cocoon’s Silk Mummy Liner ($50). A liner will extend the bag’s comfort range, or even work as a stand-alone sheet if you're somewhere warm. But mostly it will keep the bag clean.
Still, you’ll probably need to wash the bag once or twice. Try to find some down soap, like Nikwax Down Wash ($10), and use a front-loading washer, which is best for bags. If not, hand-wash in warm water with a bit of mild detergent. Washing will help restore loft, plus keep the bag smelling better.
Have a good trip!