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$τ^{9}$ Eri: A bright pulsating magnetic Bp star in a 5.95-day double-lined spectroscopic binary

$τ^{9}$ Eri is a Bp star that was previously reported to be a single-lined spectroscopic binary. Using 17 ESPaDOnS spectropolarimetric (Stokes $V$) observations we identified the weak spectral lines of the secondary component and detected a strong magnetic field in the primary. We performed orbital analysis of the radial velocities of both components to find a slightly eccentric orbit ($e= 0.129$) with a period of $5.95382(2)$ days. The longitudinal magnetic field ($B_\ell$) of the primary was measured from each of the Stokes $V$ profiles, with typical error bars smaller than 10 G. Equivalent widths (EWs) of LSD profiles corresponding to only the Fe lines were also measured. We performed frequency analysis of both the $B_\ell$ and EW measurements, as well as of the Hipparcos, SMEI, and TESS photometric data. All sets of photometric observations produce two clear, strong candidates for the rotation period of the Bp star: 1.21 days and 3.82 days. The $B_\ell$ and EW measurements are consistent with only the 3.82-day period. We conclude that HD 25267 consists of a late-type Bp star (M= $3.6_{-0.2}^{+0.1} M_\odot$, T= $12580_{-120}^{+150}$ K) with a rotation period of 3.82262(4) days orbiting with a period of 5.95382(2) days with a late-A/early-F type secondary companion (M= $1.6\pm 0.1 M_\odot$, T= $7530_{-510}^{+580}$ K). The Bp star's magnetic field is approximately dipolar with $i= 41\pm 2^{\circ}$, $β= 158\pm 5^{\circ}$ and $B_{\rm d}= 1040\pm 50$ G. All evidence points to the strong $1.209912(3)$ day period detected in photometry, along with several other weaker photometric signals, as arising from $g$-mode pulsations in the primary.