Radial velocity peak-to-peak amplitude versus the temperature class of HdC stars determined at maximum brightness. EHe stars have been given a class of −1. In the left panel, the colour code indicates the type of supergiant star. The crosses are measurements obtained from the literature (see text), while the large dots are from Gaia DR3. The names of interesting targets with low or high RV amplitude have been added. The names WISE J1942-2032 and WISE J1757-0753 are abbreviations for the RCB stars WISE J194218.38-203247.5 and WISE J175749.76-075314.9. The typical Gaia DR3 error on RV amplitude for bright stars is shown near the colour legend. Stars presenting an RVamp value lower than their respective error on RV are indicated with a question mark (see discussion in Sect. 2.2). The right panel shows the same graph but colour-coded for the number of visibility periods for the stars observed by Gaia DR3. The stars with a low number are in dark red, while the ones with more than ten visibility periods are in blue.