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We discuss as a new signature for the interaction of extragalactic ultrahighenergy protons with cosmic microwave background radiation a spectral featurelocated at $E=6.3\times 10^{19}$ eV in the form of a narrow and shallow dip. Itis produced by the interference of $e^+e^-$-pair and pion production. We showthat this dip and in particular its position are almost model-independent. Itsobservation by future ultrahigh energy cosmic ray detectors may give theconclusive confirmation that an observed steepening of the spectrum is causedby the Greisen-Zatsepin-Kuzmin effect.Comment: 4 pages, 3 figures, identical to publication in PR

Second dip as a signature of ultrahigh energy proton interactions with cosmic microwave background radiation