Tracking devices for handheld controllers

A system may include an electronic device such as a head-mounted device and a handheld input device for controlling the electronic device. A tracking device may be removably coupled to the handheld input device to increase the precision with which the head-mounted device tracks the handheld input device. The tracking device may have larger dimensions than the handheld input device and/or may have an expandable housing that expands and retracts. Visual markers on the tracking device may include infrared light-emitting diodes, geometric surface features, or other optical tracking features. The tracking device may serve as a secondary input device for the head-mounted device when the tracking device is decoupled from the input device. The tracking device may have a recess for receiving a shaft of the input device and may have magnets or other attachment structures for attaching the tracking device to the input device.