I think the key word here is 鈥渓ightweight,鈥� 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鈥檒l save a pound or more over a synthetic-fill bag.
You have solid choices. One is the Marmot Helium ($389). It鈥檚 rated to 15 degrees, has fluffy 850-fill down, and weighs just two pounds, two ounces. That鈥檚 pretty impressive, and it compresses to about the size of a cantaloupe.
I also really, really like Western Mountaineering鈥檚 Apache MF bag ($410). It鈥檚 just a touch lighter than the Helium, rated to 15 degrees, and features Western鈥檚 exceptional construction.
Another good bag is Montbell鈥檚 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鈥檝e tried it, and it works.
Two more suggestions: I highly recommend taking a silk bag liner, one like Cocoon鈥檚 Silk Mummy Liner ($50). A liner will extend the bag鈥檚 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鈥檒l 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!