A strange ellipsoidal binary ot period 0.62 days or an even stranger Delta Scuti star with a period of 0.31 days? These were the options offered by Lampens (1987, Astr. Ap. 172,173) to explain HD 96008, based on his Geneva photometry of this southern FO dwarf obtained from ESO during 1983-85. New UBVRI photometry of the star, obtained in January 1989 with the U. Toronto 0.6-m telescope on Las Campanas (Chile) -- combined with reanalysis of Lampens's original data -- give a period of 0.309873 +/- 0.000005 days and a total amplitude in V of about 0.03 mag. The light curve is clearly nonsinusoidal, with a steep descending branch and a gradual rise (see Figure 1b). Lampens' assertion of a sinusoidal variation with possible periods of 0.31 or 0.62 days arose from a misidentification of the 1 cycle/year alias of the true period (Figure 1a). The actual asymmetry of the light curve and shorter period do not match the interpretation of HD 96008 as an ellipsoidal binary with equal-mass components.
Although the colours and estimated absolute magnitude, Mv = 2.1- 2.4 (Lampens 1987; Stetson 1989, priv. comm.), place this star within the Delta Scuti instability strip, its period is longer than any recognized Delta Scuti variable and its absolute magnitude is roughly two mag too faint to obey the P-L-C relations of Breger (1979, P.A.S.P. 91, 5) and Stellingwerf (1979, Ap. J. 227 935). There is no indication of anomalous reddening, and both Geneva and Stromgren calibrations yield similar values of Mv. The shape of the light curve seems inconsistent with pulsation, but Poretti and Antonello (1988, Astr. Ap. 199, 191) have presented three examples of long-period Delta Scuti stars whose light curves also exhibit steeper descending branches.
HD 96008 is difficult to accept as a classical Delta Scuti star. Unfortunately, other possible interpretations ("starspots", a V471 Tau-type binary containing a main-sequence star and a white dwarf, an RRc Lyrae pulsator whose luminosity has been underestimated) also prove unsatisfactory. This star appears to be a rather normal Pop. I star with near-solar abundance and intermediate space motions. The next logical step to solve the puzzle will be to obtain a radial velocity curve which could be used to distinguish between pulsation and binary motions. Given the low RV amplitude expected, spectra of high S/N and moderate- to-high resolution will be necessary. Such data would also provide an accurate value of v sin i for the star and confirm the photometric constraints on its luminosity and metallicity.
Figure 1: Top Phase diagram of both the Lampens (1987) V photometry (filled circles) and the new Matthews data (open circles), plotted according to the period of 0.309603 days identified by Lampens. There is a difference in the shapes of the curves defined by the data sets, and therefore, in the time of maximum light. Bottom Same as top but for the revised period of 0.309873 days (which corresponds to a frequency very close to 1 cycle/year less than the Lampens value). Both data sets now agree very well over the six years spanned by the observations, and Lampens' data now show the same asymmetry seen in the Matthews photometry. (Full text submitted to Astronomy and Astrophysics)