I have a pair of clip-on high brightness white LED "computer lights" each normally powered by 5V from a USB port on the computer. Each is wired in series with a 15ohm 1/8W resistor. I'd like to gang them together independent of the computer and use a 9V battery for power. Where I'm getting confused is calculating what resistance to use to drop the 9V to a suitable value to handle both of them. I don't want to change the resistors tied to the LEDs; they're encapsulated and encased in a metal ferrule.
If you haven't seen these gadgets, there's a picture at:
Help would be appreciated.