Bicycle anti-theft alarm, lightweight

Does anybody know of a lightweight bicycle anti-theft alarm that could be used to supplement a cable lock? The alarm could be made up of a small-gage cable that locks into an alarm unit, and would wrap around part of the stationary object to which the bicycle is cabled. The combination of cable lock plus alarm could be a viable alternative to carrying a heavy U-type lock, for example (which I never carry because of the weight). The main use (for me) would be parking my bicycle downtown, where there are lots of people out and about.