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We present a new estimate of the deceleration parameter from the angular size-redshift test. Instead of using VLBI data to define a characteristic angular size for the sources (Kellermann 1993, Nature 361, 134; Gurvits 1994, ApJ, 425, 442), our approach uses flat-spectrum quasars and BL Lac objects to determine the high-frequency (IR-optical) break in the synchrotron spectrum. This break is associated with the synchrotron self-absorption of the most compact jet component. We derive angular sizes by making the hypotheses that these components have similar magnetic field strengths and that they are nearly stationary ($\delta \approx 1$). The method represents an alternative to the problems of limited dynamic range, small number of sources and restricted frequency coverage of the VLBI observations. It also avoids the problem of orientation of the jet axis to the line of sight.
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