As the title says - some steam VOR indicators got an ARC indicator lamp. According to the POH of an aircraft featuring such a VOR indicator this means Automatic Radial Centering. What is that ? I couldn't find an explanation anywhere.
As the name indicates, this automatically centers the CDI needle, by selecting the appropriate TO radial. From the manufacturer of INS 422, an integrated navigation system including a VOR:
Pushing the remote automatic radial centering button enables circuitry to automatically position the digital CDI indication to provide a one-time radial "TO" the VOR station. The digital indications displayed on the CDI will then return to conventional operation, permitting the radial to be flown in a conventional manner.
Without this system to determine on which radial the aircraft is flying, the pilot must turn the OBS until the instrument indicates TO and the needle is centered. With the ARC feature, this is done automatically.