DISCOVERY OF MULTIPLE PERIODICITY AND NONLINEARITY IN THE PULSATIONS OF THE ROAP STAR HD-19918

We present new high-speed photometric observations of the rapidly osci llating Ap star HD 19918 obtained at SAAO on 31 nights in 1991, 1992 a nd 1994. These observations reveal that HD 19918 pulsates with at leas t two modes which are either consecutive overtones of a single l, or m odes of alternating even and odd l. The principal frequency of oscilla tion is shown to be nu(1) = 1510.208 +/- 0.004 mu Hz, but the second f requency, nu(2), is ambiguous by 1 cycle d(-1). Removal of nu(1) and n u(2) indicates the presence of further signal content although no secu re frequency determinations are possible with the data at hand. A mult i-site campaign will be required to identify nu(2) unambiguously and t o solve the frequency spectrum fully. The first harmonic of the princi pal oscillation frequency is also present in the data, bringing to eig ht the number of rapidly oscillating Ap stars that have non-linear lig ht variations. We also present differential mean light Johnson UBVRI o bservations of HD 19918 and two comparison stars acquired on 23 nights in 1991 for the purpose of determining HD 19918's rotation period. A frequency analysis of these observations does not reveal any significa nt oscillations.