Appendices A and B are only available in electronic form at http://www.edpsciences.org

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INTErnational Gamma-Ray Astrophysics Laboratory.

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... WEBT

Whole Earth Blazar Telescope.

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... calibrators

In Sect. 3.1 we show that this assumption was true for all the observed extragalactic calibrators but 1642+690.

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... "chopper-wheel

Note that the "chopper-wheel'' calibration procedure (which at the 30 m telescope does not use a chopper wheel) is totally unrelated to the "beam-switching'' mode (which uses the chopper wheel). This paradoxical nomenclature has been maintained for historical reasons.

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... source

We assumed a negligible degree of circular polarization ( $p_{\rm {C}}$ ), which is null for the observed Galactic sources and typically <1% at cm and mm wavelengths for extragalactic AGN (Clemens Thum private communication; Homan & Lister 2006 and references therein). In particular, at 86 GHz the measured $p_{\rm {C}}$ of 0716+714 is <0.4% (Clemens Thum, private communication).

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