Photometry of four binary subdwarf B stars and the nature of their unseen companion stars

We present lightcurves of four binary subdwarf B stars, Ton 245, Feige 11, PG1432+159 and PG1017−086. We also present new spectroscopic data for PG1017−086 from which we derive its orbital period, P = 0.073d, and the mass function, fm = 0.0010±0.0002M⊙. This is the shortest period for an sdB binary measured to-date. The values of P and fm for the other sdB binaries have been published elsewhere. We are able to exclude the possibility that the unseen companion stars to Ton 245, Feige 11 and PG1432+159 are main-sequence stars or sub-giant stars from the absence of a sinusoidal signal which would be caused by the irradiation of such a companion star, i.e., they show no reflection effect. The unseen companion stars in these binaries are likely to be white dwarf stars. By contrast, the reflection effect in PG1017−086 is clearly seen. The lack of eclipses in this binary combined with other data suggests that the companion is a low mass M-dwarf or, perhaps, a brown dwarf.