THE DISCOVERY OF DELTA-SCUTI PULSATIONAL VARIABILITY IN THE PRE-MAIN-SEQUENCE HERBIG AE STAR, HR-5999, AND THE DISCOVERY OF ROTATIONAL LIGHT VARIABILITY IN THE REMARKABLE HE-WEAK BP STAR, HR-6000

We have discovered delta Scuti pulsation with a period of 4.99 h in th e well-studied pre-main-sequence Herbig Ae star, HR 5999. We present o bservations which show that we can clearly detect periodic light varia tions with a V amplitude of only 0.013 mag peak-to-peak in the presenc e of 0.35 mag of non-periodic variability caused by the well-known var iations in the obscuration by dust in the disc surrounding HR 5999. To make only the assumption of p-mode pulsation puts an immediate constr aint 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 a nd radius. The pulsation amplitude did not vary significantly between our two observing runs when the brightness of the star plus disc chang ed 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 d isc. We initially used HR 6000 as a comparison star, as have previous photometric studies of HR 5999. However, we have discovered that HR 60 00 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-w eak 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 m odels explaining the unusual nature of this star.