The plus symbols show the declination residuals of the observed position of the photocentre of Haumea’s system with respect to the theoretical position in declination, from JPL#81 ephemerides. The residuals are shown versus the date of observation. All of the observations were obtained with the La Hita 0.77-m telescope, as explained in Methods. The thin solid line represents a sinusoidal fit to the residuals, with the period determined from a periodogram analysis that is coincident with the orbital period of the moon Hi’iaka. Outlier values have not been removed. In right ascension we did not detect an oscillating behaviour of the residuals because the orbit Hi’iaka does not extend as much as in declination and the quality of the data was not good enough to show the periodicity.