Gaia DR2 number of visibility_periods_used as a function of RUWE. Old and new Ap stars without information on their binarity are plotted as small filled and open symbols, respectively. Stars described by Mathys (2017) as single or close binary stars are plotted with pluses and asterisks, respectively. Only one of the apparently single stars (HD 51684, labelled) has RUWE > 1.4. Two old Ap stars with negative parallax (HD 81009) or with only an estimated lower limit of RUWE (HD 137909) are also labelled, as are the two stars with the lowest number of visibility_periods_used (HD 9996 and 2MASS J18261274−0646548). For our selection of the best-measured stars, we used visibility_periods_used > 9, which is indicated by the dashed line.