We have discovered delta Scuti pulsation with a period of 4.99 h in the well-studied pre-main-sequence Herbig Ae star, HR 5999. We present observations which show that we can clearly detect periodic light variations with a V amplitude of only 0.013 mag peak-to-peak in the presence of 0.35 mag of non-periodic variability caused by the well-known variations in the obscuration by dust in the disc surrounding HR 5999. To make only the assumption of p-mode pulsation puts an immediate constraint on the mean density, hence on the mass and radius, of HR 5999 - a constraint which is consistent with a previous estimate of the mass and radius. The pulsation amplitude did not vary significantly between our two observing runs when the brightness of the star plus disc changed by 0.35 mag in V. This supports the model that the long-term, large-amplitude variations are caused by variable dust obscuration in the disc. We initially used HR 6000 as a comparison star, as have previous photometric studies of HR 5999. However, we have discovered that HR 6000 is variable in V light by 0.008 mag peak-to-peak with a period near to 2 d. This is plausibly the rotation period of this remarkable He-weak Bp star. If so, the measured upsilon sin i less than or equal to 5 km s(-1) argues for a pole-on orientation. That may have bearing on models explaining the unusual nature of this star.
