A photometric study of southern Ap stars having right ascensions on the order of 12 h

From uvby measurements periods have been deduced for the Ap stars 25 Sex, HR 4327, β Hya, HD 112381, HR 4965, HR 5158, HR 5269, and HD 125630. The star HR 4109 (Sr?) remained constant during an interval of 23 d; the period is perhaps very long, or if HR 4109 actually does not vary, possibly it is not an Ap star. The observed variation for l Cen (Mn) was too weak to deduce a period. The variation range of HD 112381 (SiCr) is nearly as small as that of l Cen. The v and y variations for HD 125630 (SiCrSr), which are double-waved and nearly opposite to one another, have ranges among the largest known for Ap stars; the c[SUB]1[/SUB] range is also exceptionally large. For some Ap stars the variation curves show a dip of short duration in the four colours, and for a few stars a second one appears at a phase-interval of P/2. The nature of this phenomenon is discussed.