Ultraviolet photometry from the Orbiting Astronomical Observatory. XV. The strongly magnetic variable HD 215441.

The Wisconsin Experiment Package on OAO-2 was used to obtain light curves of the strongly magnetic, variable Ap star RD 215441 over the wavelength range 155() 4250 A. This star is characterized by a lack of outstanding spectrum variability at wavelengths accessible from the ground, while undergoing UB V photometric variations of large amplitude. The vacuum ultraviolet data show that the UB V variations are driven by a time-variable redistribution of flux from the ultraviolet to the visible (variable backwarming), similar to the mechanism previously noted by Molnar in the well-known spectrum variables a2 CVn and e UMa. At 2980 A RD 215441 varies in phase with the visible; the data firmly establish 2460 A as a "null wavelength" at which the star does not vary systematically in brightness with phase; shortward of 2460 A the star reaches minimum brightness at the time of visible maximum. The effective temperature of RD 215441, crudely estimated as 14,5000 K, does not vary by more than a few hundred degrees over the star's cycle and may well be constant as a function of phase. The observations of RD 215441 demonstrate that an Ap star need not be a striking spectrum variable at blue-visible wavelengths for its photometric variations to be controlled by opacity variations in the ultraviolet. Subject headings: atmospheres, stellar magnetic stars - stars, individual - spectrum variables - ultraviolet