Kevin Krisciunas (Joint Astronomy Center, Hilo, Hawaii) reports 9 Aur (BS 1637) has a spectral type of FOV (Bright Star Catalog) or F1 IV/IV-V (by Gray and Garrison 1989). Abt (1965) found it to be a single-lined binary, but a subsequent paper by Abt and Levy (1974) "did not confirm the previous orbit". If we believe the uncertainties of the radial velocities of the latter paper as well as data by Takeda (1984), there are radial velocity variations of 5 or 6 km/sec. From speckle data Hartkopf and McAllister (1984) did not find evidence for a close companion at the 0.030 arcsec level. (According to Gliese-s catalog, 9 Aur has a 12th magnitude companion 5 arcsec away, and a 9th magntitude companion 90 arcsec away.)
My interest in 9 Aur comes from using it as a comparison star for photometry of Capella. My differential photometry of the two stars obtained during 14 nights from 1987 January through 1988 November exhibits a repeatable quasi- sinusoidal light curve of amplitude dV = 0.07 mag and a period of 39.4 days. At first I thought that I might have demonstrated the variability of Capella. However, this period does not match the expected rotational periods of either Capella component (spectral types G6 and F9 lll, respectively). Ed Guinan could not find such variations nor any repeatable variability of Capella while using BS 1668 as a comparison star. Naturally, one must suspect 9 Aur as the variable.
Since 1988 September I have observed 9 Aur vs. BS 1668 on 16 nights, obtaining 74 differential V magnitudes. The nightly means indicate a range of 0.10 mag in V. The 39.4 day period is not confirmed by the nightly means.
Instead, the data of 1989 September4 to 1988 November 9 (UT) indicatea best period of 33.8 minutes, while the data of 1989 December 5 are fit better with a period close to one hour. Clearly, more data is required with more data points per night.
One can rule out star spots as a cause of a 39.4 day period. The small v sin i value of 14 km/sec gives a maximum rotation period of about 5 days if 9 Aur is an F0 dwarf.
To test the possibility that the 39.4 day period was due to a lumpy ring of dust orbiting 9 Aur, we observed it together with two other F0V stars (BS 1869 and BS 2228) with the United Kingdom Infrared Telescope, obtaining photo- metry from 1.2 to 4.7 microns. There was no evidence of an infrared excess at 3 to 4 microns for any of the three stars. One would have expected an infrared excess in the 3 to 4 micron range from dust at the distance required for a 39.4 day orbit around an FOV star.
My best guess is that 9 Aur is exhibiting some pulsation (as also shown by the radial velocity data) . It lies on the cool border of the instability strip in the HR Diagram. 9 Aur should not be used as a photometric standard.
Phase diagram of the differential V photometry of 9 Aur relative to BS 1668. The symbols correspond to 1989 UT dates of Sept. 4 (open circles), Sept. 12 (squares), Sept. 21 (triangles), and Nov. 9 (dots).