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Abstract

We investigated the radial velocity (RV) and light variations of the rapidly oscillating Ap (roAp) star γ Equ( HD 201601, HR 8097), with simultaneous (3.6m CFHT) spectroscopic and (UH 0.6m) photometric observations taken over 4 nights in August 1988. Despite lowamplitude detection limits on all nights, convincing RV and light oscillations were seen only on the last night, confirming amplitude modulation and/or mode evolutions noted by previous observers. The RV variations for the last night had a dominant frequency of 1.517 ± 0.01 mHz (P ~ 11.0 minutes) with an amplitude of 418 m s⁻¹, differing significantly from the 1.365 mHz (amplitude ~ 0.001 mag, P ~ 12.2 minutes) photometric frequency of the same night. We attribute the two frequencies to different modes observed almost equator on: the 1.5 mHz frequency to an (l=2, m=0) mode, and the 1.-4 mHz frequency to a dipole (l=1,m=0) mode. The former mode would have high RV amplitude but low photometric amplitude at this orientation, explaining the lack of photometric signal. This is the first time a non-dipole mode has been identified i n a roAp star. Based on our frequency data and those of other investigators plus the mode identifications above, we uniquely determine a fundamental p-mode spacing, Δv[sub o] ~ 60 μHz. Combined with stellar models, this value implies that γ Equ is near the main sequence, resolving an ambiguity about this star which has existed for almost a decade. Our results indicate it is critical that investigators use modes of the same degree l and orientation when determining the RV-to-light ratio (2K/Δm ) if it is to be used as a diagnostic of temperature and pulsational characteristics in roAp, stars.