5 HD 115708

Wade et al. (1996) found that the longitudinal component of the magnetic field of the SrEu CP star HD 115708 varies with the ephemeris

JD (positive magnetic maximum) = 2435997.50 + 5.07622 $\pm$ 0.0004 E

which is consistent with the period found by Leroy (1995) and also takes into account observations taken by Babcock (1958). Photometric variability was found by Wolff (1975) using Strömgren photometry.

The FCAPT obtained 35, 46, and 31 good sets of differential Strömgren values during the 1995-96, 1996-97, and 1997-98 observing seasons, respectively. Their Scargle periodograms suggests a period close to 5.076 days or its alias. When this data and that of Wolff (appropriately rezeroed) are plotted with the ephemeris of Wade et al. (Fig. 2), they fall on top of one another. Thus the light curves have the same shape for both eras. When one considers of the effects of slight differences in the period, the quoted error in the magnetic period is reduced by a factor of 4.

  

Figure 2: Photometry of HD 115708 plotted according to the ephemeris of Wade et al. (1996) JD (positive magnetic maximum) = 2435997.50 + 5.07622 E with FCAPT values being given as closed squares and those of Wolff (1975) as plus signs

Minimum light occurs at phase 0.5 for the u and b light curves. A secondary minimum is near phase 0.9. The amplitudes of variation are 0.025 mag in u, 0.035 mag in v, 0.015 mag in b, and 0.02 mag in y. The light curves for u and v are in phase, but out of phase with y and probably b as far as the u and v light minima are concerned, but the b and y light minima coincide with u and v light secondary minima. The phases of the magnetic extrema are 0.43 and 0.93 which are close to those for light variability. This indicates very complex surface abundance distributions. For much of the period there is probably a nearly non-variable wavelength region between the mean wavelengths for the v and b filters.

Acknowledgements

This work was supported in part by NSF grants AST-9115114 and 9528506 and in part by grants from The Citadel Development Foundation. I appreciate the continuing efforts of L.J. Boyd, R.J. Dukes Jr., and G.P. McCook to keep the FCAPT operating properly and the useful comments of the referee G.A. Wade.