  We describe a double-peak structure of the coincidence spectrum of biphoton
states in the process of spontaneous parametric down-conversion with a pump
having the form of short pulses. The effect is shown to arise owing to the
obligatory symmetry of bihoton wave functions, as wave functions describing
states of two bozons obeying the Bose-Einstein statistics. Parameters of the
peaks are found and conditions necessary for experimental observation of the
effect are determined.
