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We have performed quasi-simultaneous radio Ñux density measurements at 2.7 and 10 GHz for all Palomar-Green (PG) quasars with radio Ñux densities between 4 and 200 mJy. We Ðnd that a large fraction of these sources are variable, Ñat-spectrum quasars. This brings to 40% the total fraction of Ñat-spectrum quasars with a radio-to-optical Ñux ratio R [ 10 in the PG quasar sample. We also Ðnd that the median R-parameter of these Ñat-spectrum quasars is lower than those of steep-spectrum radioloud quasars. This contradicts the predictions of the uniÐed scheme and the idea that all Ñat-spectrum, core-dominated quasars are relativistically boosted lobe-dominated quasars. We show that this discrepancy is due to a population of Ñat-spectrum radio-intermediate quasars with 25 \ R \ 250 that can be explained neither as relativistically boosted radio-loud quasars nor as normal radio-weak quasars. We point out that a natural explanation for the Ñat-spectrum radio-intermediate quasars is relativistic boosting in radio-weak quasars. If the Ñat-spectrum radio-intermediate quasars are considered the boosted counterparts to usual radio-weak quasars, their fraction among radio-weak quasars is roughly 10%, similar to the fraction of boosted radio-loud quasars. This would point toward average Lorentz factors of for radio-loud and radio-weak quasars. The presence of the Ñat-spectrum radio-intermediate c jet \ 2È4 quasars changes the deÐnition of "" radio-loud ÏÏ and can bias some conclusions drawn from optically selected quasar samples, where R^1È10 is used as the dividing line for both Ñat-and steep-spectrum quasars. Instead, one should use separate R-parameters for the dividing line in steep-spectrum (R^25) and Ñat-spectrum (R D 250) quasars.

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We present a catalogue of 781 compact radio sources in the declination range 0° ≤ δB1950≤ +20° whose positions have been measured to an rms accuracy of about 14 mas with the Very Large Array (VLA). These sources are primarily intended for use as phase calibration sources for the Jodrell Bank MERLIN. However, they will also be suitable as phase calibrators for the VLA and can be considered as candidate phase calibrators for very long‐baseline interferometry (VLBI) networks.

A B S T R A C TWe present a catalogue of 540 compact radio sources in the declination ranges þ20Њ Յ d B1950 Յ þ35Њ and þ75Њ Յ d B1950 Յ þ90Њ, the positions of which have been measured to an rms accuracy of about 55 mas with the Very Large Array (VLA). These sources are primarily intended for use as phase calibration sources for MERLIN. However, they will also be suitable as phase calibrators for the VLA and can be considered as candidate phase calibrators for very long baseline interferometry (VLBI) networks.