New Photometry of the Hyades

71 Tauri (= BS 1394 = vB 141 = V777 Tau) is a rapidly rotating, F0 IV–V member of the Hyades cluster. It is the second brightest X-ray source in the cluster, after V471 Tauri, and an intensely bright coronal EUV source (Stern et al. 1995). It is a lunar occultation binary, but the secondary star (estimated by Peterson et al. 1981 to be a G4 V star in possibly a 53 day orbit) has never been seen directly. As pointed out by Stern et al., the tremendous X-ray luminosity of 71 Tau, L x = 10 30 erg s −1 , is not the result of a flare. It is not easily explained as the coronal emission of an individual star, or even as the combination of emission from several stars, considering that most F–M Hyades stars are detected at levels of only L x ∼ few ×10 29 erg s −1 (or in some cases, much less). The X-ray properties of 71 Tau are more in line with those of a very much younger star, but would still be considered remarkable even for a coronally active star in the ≈80 Myr old Pleiades cluster (Stauffer et al. 1994). 71 Tau has been observed a number of times by IUE and has been shown to be variable by as much as 30 % at ultraviolet wavelengths near 1700–2000Å (Simon and Landsman, in preparation). Horan (1979) discovered that 71 Tau was a δ Scuti star with an amplitude of 0.01 to 0.02 mag. To our knowledge no other optical photometry of 71 Tau has ever been published. Horan gives a principal period of 3.9 hours (f = 6.15 d −1) and suggests that 71 Tau may be excited in more than one mode. However, his conclusions are based on only 38 points obtained on two nights which were two months apart. From Horan's Figure 5 it seems much more likely that the principal period is near 4.4 hours if the decline in brightness occurs at the same rate at the increase of brightness. We have obtained new photometry of 71 Tau, observed differentially with respect to BS 1422, amounting to 58 points on 5 nights during a 7 night run with the 0.6-m telescope at Mauna Kea. Observations of BS 1432 (the check star) vs. BS 1422 indicated that both were constant, so any variations in the differential magnitude of …