2004MNRAS.351..685O

We confirm the orbital period of WX Cen ≡ WR 48c determined by Diaz & Steiner and refined its value to P_orb= 0.416961 5(±22) d. The light curve of this object has a peak-to-peak variation of approximately 0.32 mag. It is non-sinusoidal in the sense that it has a V-shaped narrow minimum, similar to the ones seen in V Sge, V617 Sgr and in compact binary supersoft sources (CBSS).

Most of the emission lines in the optical spectrum are due to Balmer, HeII, CIV, NV, OV and OVI. An analysis of the HeII Pickering series decrement shows that the system has significant amount of hydrogen. The emission lines of HeII 4686 Å became weaker between the 1991 and 2000/2002 observations, indicating distinct levels of activity. The spectra of WX Cen show variable absorption features in the Balmer lines with V=-2900 km/s and in emission with V=±3500 km/s. These highly variable features remind us of the satellite emission lines found in the spectra of CBSS.

We estimate the colour excess as E(B-V) = 0.63 on the basis of the observed diffuse interstellar band at 5780 Å. Given the distance-colour excess relation in the direction of WX Cen, this implies a distance of 2.8±0.3 kpc. Interstellar absorption of the Nai D lines show components at -4.1 km/s, which corresponds to the velocity of the Coalsack, and three other components at -23.9, -32.0 and -39.0 km/s. These components are also seen with similar strengths in field stars that have distances between 1.8 and 2.7 kpc. The intrinsic colour of WX Cen is (B-V)₀=-0.2 and the absolute magnitude is M_V=-0.5.

Extended red wings in the strong emission lines are seen. A possible explanation is that the system has a spill-over stream similar to what is seen in V617 Sgr. We predict that when observed in opposite phase, blue wings would be observed. A puzzling feature that remains to be explained is the highly variable red wing (V∼ 700 km/s) of the Ovi emission lines as well as of the red wings of the H and He lines.

The velocity of the satellite-like feature is consistent with the idea that the central star is a white dwarf with a mass of M∼ 0.9 M_☉. With the high accretion rate under consideration, the star may become a Type Ia supernova (SN Ia) in a time-scale of 5x10⁶ yr.