Abstract: An optical data carrier has a memory member having a radiation-transparent layer and an information recording layer adjoining it. The information recording layer has a surface with memory areas for digital information. The data carrier has a through opening for receiving a read/write device and has a thickness equal to that of a chip card or magnetic strip card. The memory member is 0.5-0.6 mm thick. An adapter for the read/write device has a base member with a rectangular recess of inner dimensions slightly greater than the outer dimensions of the plate-shaped data carrier to be received therein. The base member has a central opening coaxial to the through opening of the data carrier. The adapter/data carrier unit can rotate without balance error about a central axis of through opening and central opening. The combined thickness of the data carrier and of the base member measured at the recess corresponds to that of a standard DVD data carrier.