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HD 209458

G. W. Henry, Tennessee State University; G. Marcy, University of California at Berkeley; R. P. Butler, Department of Terrestrial Magnetism, Carnegie
Institution of Washington; and S. S. Vogt, Lick Observatory, report: “The G0 V star HD 209458 = BD +18 4917 (R.A. = 22h03m10s.78, Decl. =
+18o53’03”.5, equinox 2000.0; V = 7.65) exhibits sinusoidal velocity variations with a semiamplitude of 81 m/s, indicating the presence of a companion with
M sin i = 0.63 Jupiter mass and an orbital period of 3.523 days.

Photometry reveals a transit ingress on Nov. 8.20 UT with a depth of 0.017 mag, consistent with the transit time predicted from the velocities. Further
measurements of transits and velocities would be valuable. The next three predicted times of ingress occur at Nov. 15.263, 18.787, and 22.311 (all times
uncertain by 1 hr).

If correct, the inferred mass is 0.63 that of Jupiter, and radius 1.6 that of Jupiter, implying a density of 0.21 g/cm^3. HD 209458 resides at a distance of 47 pc,
with rotational v sin i = 3 km/s, and is chromospherically inactive, with an estimated age of 4.5 Gyr.”