[en] The Optical Fiducial Volume Trigger system (OFVT) has been developed for use with the Rapid Cycling Bubble Chamber (RCBC), serving as a vertex detector in the European Hybrid Spectrometer (EHS). Its purpose is to decide whether an interaction took place within the useful (fiducial) volume of the chamber. In this way it complements the main interaction trigger which lacks the necessary accuracy of vertex localisation. The system can reduce the cost of film material, film treatment and scanning by up to 60%, the precise saving being dependent on the experimental conditions. The principle of the optical track detection by linear diode arrays is described, as well as details of the hardware construction. The properties of the system are discussed on the basis of results from the first run with RCBC. (orig.)