The EC 14026 stars - VIII. PG 1336 - 018: A pulsating sdB star in an HW Vir-type eclipsing binary

The sdB star PG 1336 - 018 is found to be a very short-period eclipsing binary system, remarkably similar to the previously unique system HW Vir. In addition, and unlike HW Vir, the sdB star in the PG 1336 system shows rapid oscillations of the type found in the recently discovered sdB pulsators, or EC 14026 stars. The orbital period, 0.101 0174 d, is one of the shortest known for a detached binary. Analysis of photoelectric and CCD photometry reveals pulsation periods near 184 and 141 s, with semi-amplitudes of ∼0.01 and ∼0.005 mag respectively. Both oscillations might have variable amplitude, and it is probable that other frequencies are present with amplitudes ∼0.003 mag or less. The 184- and 141-s pulsations are in the range of periods predicted by models for hot horizontal-branch stars. Analysis of medium-dispersion spectrograms yields Teff = 33 000 ± 1000 K and log g = 5.7 ± 0.1 for the sdB primary star, a radial velocity semi-amplitude K1 = 78 ± 3 km s-1 and a system velocity γ = 6 ± 2 km s-1. Spectrograms from the IUE Final Archive give Teff = 33 000 ± 3000 K and E(B - V) = 0.05 for log g = 6.0 models. The derived angular radius leads to a distance of 710 ± 50 pc for the system, and an absolute magnitude for the sdB star of +4.1 ± 0.2. A preliminary analysis of U, V and R light curves indicates the orbital inclination to be near 81° and the relative radii to be r1 = 0.19 and r2 = 0.205. Assuming the mass of the sdB primary to be 0.5 M⊙ leads to a mass ratio q = 0.3 for the system, and indicates that the secondary is a late-type dwarf of type ∼M5. As with HW Vir, it is necessary to invoke small limb-darkening coefficients and high albedos for the secondary star to obtain reasonable fits to the observed light curves.