VLA, Metsähovi, IRAM, Effelsberg and Planck measurements of two known GPS blazars: J1800+7828 (left) and J0423–0120 (right). While the former shows only small signs of variability, the latter is clearly variable while preserving its spectral shape, pointing to helical or precessing jet (see text). Asterisks identify epochs within 10 days from Planck observations at any of the LFI channels. For both sources, the Planck ERCSC data are a superposition of two scans separated by 3 and 6 months, respectively.